Back pain accounts for about one fourth of workers' compensation claims in the United States. The Occupational Health Supplement to the 1988 National Health Interview Survey provided an opportunity to assess the scope of this problem. The 30,074 respondents who worked in the 12 months before the interview were defined as "workers", and those with back pain every day for a week or more during that period were defined as "cases." A weighting factor was applied to the answers to derive national estimates. In 1988, about 22.4 million back pain cases (prevalence 17.6%) were responsible for 149.1 million lost workdays; 65% of cases were attributable to occupational activities. For back pain attributed to activities at work, the risk was highest for construction laborers among males (prevalence 22.6%) and nursing aides among females (18.8%). Our analyses show that back pain is a major cause of morbidity and lost production for U.S. workers and identifies previously unrecognized high risk occupations, such as carpenters, automobile mechanics, maids, janitors, and hairdressers, for future research and prevention.