OBJECTIVE: Back pain is a leading cause of absence and disability in the workplace. Costs associated with losses in productivity due to back pain are significant and may be as high as the costs of medical care for this condition. However, there has been limited study of the indirect costs of back pain in the United States. The objective of this study was to determine the indirect costs due to back pain in the U.S. population. METHODS: A retrospective analysis of the 1996 portion of the Medical Expenditure Panel Survey (MEPS) was conducted. The MEPS provided data from a nationally representative sample of 22,601 respondents and data from respondents' medical care and health insurance providers and employers. Data included medical conditions and employment information comprised of hourly earnings, hours worked, and disability days. Back pain patients who incurred disability days were identified using International Classification of Diseases (ICD-9-CM) codes determined by an expert panel of physicians and coders as indicative of back pain and variables denoting disability days. Indirect costs were calculated for back pain patients who missed workdays using the human capital approach. Sample estimates were weighted and projected to the population and 95 percent confidence limits for estimates were calculated using the Taylor expansion method. RESULTS: Total indirect costs for back pain patients who missed workdays were $18,533,583,620. Mean indirect costs were $4,586 per back pain patient who missed workdays (95% C.L.=$3,852 to $5,321). Relative to the entire population, the mean indirect costs per person were $68.92. CONCLUSIONS: With losses in productivity greater than $18.5 billion, indirect costs significantly contribute to the total costs of back pain. The indirect costs of back pain to labor underscore the need to prevent back injury thereby promoting employee health and maintaining productivity.