OBJECTIVES: The objectives of this study were to determine and compare the prevalence and direct costs of back pain in the U.S. in 1996 and 1999. METHODS: Retrospective analysis was conducted of the 1996 and 1999 portions of the Medical Expenditure Panel Survey (MEPS). The MEPS collected data from nationally representative samples of 22,601 respondents (1996) and 24,618 respondents (1999) and from respondents' health care and insurance providers. Data used for this study included medical conditions and use and payments for medical care. Back pain patients were identified using ICD-9-CM codes determined by an expert panel of physicians and professional coders as indicative of back pain. Direct costs were calculated using patient and third-party payments for back pain related medical events by type of medical care. Sample estimates were weighted and projected to the population and 95% confidence limits were calculated using the Taylor expansion method. RESULTS: There was no significant difference in the prevalence of back pain between 1996 (8.74%) and 1999 (9.00%). From 1996 to 1999, total direct costs increased from $14,701,417,650 to $25,847,917,882. Total inpatient stay expenditures represented the largest proportion of this increase in direct costs, increasing from $4,658,655,867 (mean=$10,016; 95% CL=$9,463-$10,579) to $12,809,062,318 (mean=$17,305; 95% CL=$4,762-$29,847). Total expenditures on home health care services increased from $725,019,395 (mean=$395; 95% CL=$140-$650) to $3,049,290,075 (mean=$1,802; 95% CL=$618-$2,987). Total prescription medication expenditures increased from $941,406,990 (mean=$24; 95% CL=$18-$29) to $1,202,568,785 (mean=$34; 95% CL=$30-$37). Expenditures on office-based medical provider and emergency department visits showed minor increases while those on outpatient services exhibited a minor decrease. CONCLUSIONS: From 1996 to 1999, there was no difference in the prevalence of back pain however there was an $11.1 billion increase in direct costs. Inpatient stays, home care services, and prescription medications accounted for the majority of this increase.