There are no national data on the extent of back problems in the population of the United States, but it is known that back symptoms is the second leading symptomatic reason expressed by patients for visiting physicians. To provide insight into the scope of this problem, data from the National Ambulatory Medical Care Surveys of 1977 and 1978 were examined using the sex of the patient and the physician's degree (MD or DO) as control variables, and typical encounter characteristics as dependent variables. Males 45-64 years of age had the highest visit rate, and visit rates for men 15-64 years of age were higher than those of women the same age. Common diagnoses were sprains and strains, arthritis and rheumatism, displacement of intervertebral disc, and diseases of urinary tract, with men more likely than women to have injuries. DOs were more likely to treat accidental injuries than were MDs. It is recommended that differential diagnosis be taken into account before studying sex differences in complaints.