Objective-The objective of the EPISER study was to estimate the prevalence of rheumatoid arthritis (RA), low back pain, hand and knee osteoarthritis (OA), and fibromyalgia in the adult Spanish population, and to assess the impact of these diseases on function and quality of life, and use of health and social resources. Methods-2998 subjects aged 20 years or above were randomly selected by stratified multistage cluster sampling from the censuses of 20 municipalities. Trained rheumatologists carried out structured visits at which subjects were asked about rheumatic symptoms and sociodemographic characteristics, completed validated instruments for measuring function (HAQ) and quality of life (SF-12), and underwent a standardised physical examination. Cases were defined by previously validated criteria. Results-The estimated prevalences with 95% confidence intervals were as follows: RA lifetime cumulative: 0.5% (0.3 to 0.9); low back pain: 14.8% (12.2 to 17.4); symptomatic knee OA: 10.2% (8.5 to 11.9); hand OA: 6.2% (5.9 to 6.5); fibromyalgia: 2.4% (1.5 to 3.2). Most conditions significantly impaired function and quality of life. Conclusions-The EPISER study has internal and external validity for application of the results to the adult Spanish population. The diseases studied aVect a significant proportion of the population, with various degrees of impact on disability and quality of life resulting in a significant number of physician visits, work disability, and medication use. (Ann Rheum Dis 2001;60:1040-1045 Musculoskeletal diseases are one of the main causes of disability in the developed world and consume a large amount of health and social resources. The proportion of the population disabled by rheumatism ranges from 2.8% in the United States to 8% in Great Britain. [1][2][3] Nevertheless, for reasons that are not clear, these conditions are not commonly the target of epidemiologists and thus epidemiological studies on the occurrence and impact of musculoskeletal diseases compared with diseases aVecting the cardiovascular or pulmonary systems, for instance, are infrequent.Most of the epidemiological data on rheumatic diseases at the national level come either from a small number of sampling areas considered representative of the country or are secondary data from national health surveys in which information about rheumatic diseases relies mainly on self-reporting.