We conducted a prospective observational study during the winter of 1994-1995 in a representative national sample of 2,406 patients aged 18 to 65 years seen in primary care settings for acute low back pain of less than 48 hours' duration. The following data were collected: demographic, social and clinical characteristics at inclusion; treatments prescribed throughout the episode, functional impairment and restriction of usual activities. These data were used to evaluate direct costs (health costs) and indirect costs (sickness payments). The sex ratio was predominantly male (60.4%), 80% of patients were economically active, mean age was 43 +/- 11.7 years, and most patients (76%) had a history of low back pain. Management consisted primarily of rest (bedrest, 32%; rest at home, 61%) and pharmacotherapy (mainly analgesics, nonsteroidal antiinflammatory drugs and muscle relaxants; mean number of drugs per patient, 3.2). Imaging studies were obtained in 34% of cases and physical therapy was prescribed in 30%. Referral and hospitalization rates were 5.4% and 0.8%, respectively. Among economically active patients, 82% were put on sick leave, for a mean duration of 8.4 +/- 4 days and 18.6% were reported as having work-related low back pain. The mean cost of outpatient care for the episode of low back pain was 1,021 French francs (FF), most of which was contributed by physical therapy (41.6%), physicians' fees (23.9%) and investigations (16%). Mean sickness payments were 821 FF per patient in the economically active subgroup and 523 FF per patient in the overall study population.