Back pain is the most common reason for filing workers' compensation claims and often causes lost workdays. Data from the 1988 National Health Interview Survey were analyzed to identify high-risk industries and to estimate the prevalence of work-related back pain and number of workdays lost. Analyses included 30074 respondents who worked during the 12 months before the interview. A case patient was defined as a respondent who had back pain every day for a week or more during that period. The prevalence of lost-workday back pain was 4.6%, and individuals with work-related cases lost 101.8 million workdays owing to back pain. Male and female case patients lost about the same number of workdays. Industries in high-risk categories were also identified for future research and intervention, including those seldom studied. This study provides statistically reliable national estimates of the prevalence of back pain among workers and the enormous effect of this condition on American industry in terms of lost workdays.