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This research was supported by the Intramural Research Program of the NIH, National Institute of Environmental Health Sciences. None of the authors has any conflict of interest to declare.

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There are many studies based on self-reported menstrual cycle length, yet little is known about the validity of this measure. The authors used data collected in 1990 from 352 women born in Chicago, Illinois, aged 37-39 years. Women reported their usual cycle length and behavioral and reproductive characteristics at study enrollment and then completed daily menstrual diaries for up to 6 months. The authors compared this observed cycle length (geometric mean) with the reported length by using kappa coefficients. To assess systematic effects, they performed linear regression of the difference between reported and observed cycle length. Agreement between observed and reported cycle length was moderate. The crude overall kappa coefficient was 0.33; the kappa adjusted for within-woman sampling variability was 0.45 (95% confidence interval: 0.36, 0.55). On average, women overestimated their cycle length by 0.7 days (95% confidence interval: 0.3, 1.0). Reporting by sexually active women and women with a history of infertility was more accurate. Parity, body mass index, prior medical evaluation for irregular cycles, and exercise were all associated with systematic reporting differences. Studies that rely on self-reported cycle length could be prone to artifactual findings because of systematic covariate effects on reporting.

Objective-Because studies suggest that ultraviolet radiation (UVR) modulates myositis phenotype and Mi-2 autoantigen expression, we conducted a retrospective investigation to determine if UVR may influence the relative prevalence of dermatomyositis and anti-Mi-2 autoantibodies in the United States.Methods-We assessed the relationship between surface UVR intensity in the state of residence at the time of onset with the relative prevalence of dermatomyositis and myositis autoantibodies in 380 myositis patients from referral centers in the U.S. Myositis autoantibodies were detected by validated immunoprecipitation assays. Surface UVR intensity was estimated from UV index data collected by the U.S. National Weather Service.Results-UVR intensity was associated with the relative proportion of patients with dermatomyositis (odds ratio [OR] 2.3, 95% confidence interval [CI] 0.9-5.8) and with the proportion of patients expressing anti-Mi-2 autoantibodies (OR 6.0, CI 1.1-34.1). Modeling of these data showed that these associations were confined to women (OR 3.8, respectively) and suggests that gender influences UVR effects on autoimmune disorders. Significant associations were not seen in men, nor were UVR levels related to the presence of anti-synthetase or anti-signal recognition particle autoantibodies.Conclusion-This first study of the distribution of myositis phenotypes and UVR exposure in the United States showed that UVR may modulate the clinical and immunologic expression of autoimmune disease in women. Further investigation of the mechanisms by which these effects are produced may give insights into pathogenesis and suggest therapeutic or preventative strategies.Current evidence suggests that autoimmune diseases result from environmental exposures in genetically susceptible individuals and ultraviolet radiation (UVR) is an environmental exposure of increasing interest (1). Beyond UVR's recognized association with skin cancer are other immunomodulatory effects possibly related to the development of immune- Furthermore, studies suggest that UVR regulates levels of the dermatomyositis-specific Mi-2 autoantigen via protein translational effects (4).The idiopathic inflammatory myopathies (IIM) are acquired systemic autoimmune conditions that share chronic muscle weakness due to chronic muscle inflammation. The two major clinical groups, dermatomyositis (DM) and polymyositis (PM), are distinguished by the presence of photosensitive pathognomonic rashes in DM (5). These two forms of IIM share some genetic risk factors but differ in others and appear to have distinct pathogeneses (5). Categorizing myositis patients by the presence or absence of myositis-specific autoantibodies (MSA) results in more homogenous groups in terms of epidemiology, clinical presentations, genetics and prognoses (5). MSA include autoantibodies directed against aminoacyl-tRNA synthetases (anti-synthetases), the signal recognition particle (anti-SRP), and autoantibodies that react with a 240 kD protein called chromodomain helicase DNA bi...