ABSTRACT. Objective. To examine the attitudes, beliefs, and behaviors of parents whose children were underimmunized with respect to >2 vaccines that have recently received negative attention, compared with parents whose children were fully immunized with respect to the recommended vaccines.Design. Case-control study. Setting. A sample of households that participated in the National Immunization Survey were recontacted in 2001.Main Outcome Measure. Vaccination status was assessed. Case subjects were underimmunized with respect to >2 of 3 vaccines (diphtheria-tetanus-pertussis or diphtheria-tetanus-acellular pertussis, hepatitis B, or measlescontaining vaccines), and control subjects were fully immunized.Results. The response rate was 52.1% (2315 of 4440 subjects). Compared with control households, case households were more likely to make $0 to $30 000 (adjusted odds ratio [OR]: 2.7; 95% confidence interval [CI]: 1.5-4.6) than at least $75 000, to have >2 providers (OR: 2.0; 95% CI: 1.3-3.1) than 1, and to have >4 children (OR: 3.1; 95% CI: 1.5-6.3) than 1 child. With control for demographic and medical care factors, case subjects were more likely than control subjects to not want a new infant to receive all shots (OR: 3.8; 95% CI: 1.5-9.8), to score vaccines as unsafe or somewhat safe (OR: 2.0; 95% CI: 1.2-3.4), and to ask the doctor or nurse not to give the child a vaccine for reasons other than illness (OR: 2.7; 95% CI: 1.2-6.1). Among case subjects, 14.8% of underimmunization was attributable to parental attitudes, beliefs, and behaviors.Conclusions. Attitudes, beliefs, and behaviors indicative of vaccine safety concerns contribute substantially to underimmunization in the United States. Although concerns were significantly more common among parents of underimmunized children, many parents of fully immunized children demonstrated similar attitudes, beliefs, and behaviors, suggesting a risk to the currently high vaccination levels. Efforts to maintain and improve immunization coverage need to target those with attitudes/beliefs/behaviors indicative of vaccine safety concerns, as well as those with socioeconomic and health care access problems. Pediatrics 2004;114:e16 -e22. URL: http://www.pediatrics.org/cgi/content/full/114/1/e16; underimmunization, attitudes, beliefs, behaviors.ABBREVIATIONS. DTaP, diphtheria-tetanus-acellular pertussis; DTP, diphtheria-tetanus-pertussis; MCV, measles-containing vaccine; MMR, measles-mumps-rubella; NIS, National Immunization Survey; OR, odds ratio; CI, confidence interval. I mmunizations have reduced the incidence of vaccine-preventable disease by Ͼ95% for every pediatric vaccine recommended for routine use before 1990. 1 As the number of immunizations has increased, however, reports of postimmunization adverse events, both vaccine-related and coincidental, have increased. This increase, combined with the decrease in the incidence of vaccine-preventable diseases, has resulted in an increased focus on vaccine safety. 2 Some have linked vaccinations with acute and chronic illnes...