Spinal trabecular bone mineral content was measured in the first, second, and third lumbar vertebrae by quantitative computed tomography in 88 patients with non-steroid treated rheumatoid arthritis. Results were compared with those obtained in 105 healthy control subjects. The mean bone mineral content in the patient group, 135.8 (SD 32.8) mg/ml K2HOP4, was significantly lower than that in the controls (151.9 (32.1) mg/ml, p less than 0.01). Division of patients and controls into three age groups showed that the reduction in bone mineral content was most marked in the youngest age group (21-40 years), the mean value in male patients being significantly lower than in controls (149.6 (51.3) v 171.7 (23.9) mg/ml K2HPO4, p less than 0.05); in female patients in this age group the corresponding values were 160 (26.1) v 178.4 (22.0) mg/ml, 0.05 less than p less than 0.1). No significant difference in mean values between patients and controls was found in the other age groups. Of the 88 patients, six (7%) had abnormally low values, defined as a bone mineral content greater than 2 SD below the normal mean. One vertebral crush fracture was found in one patient but not in any of the controls. No correlation was found between bone mineral content and body weight, duration of disease, or disability as assessed by the London and Steinbroker methods. These results demonstrate a lower spinal trabecular bone mineral content in non-steroid treated patients with rheumatoid arthritis than in age and sex matched controls, the difference being most marked in younger patients. The finding of abnormally low values in 7% of the patients indicates a slightly increased prevalence of spinal osteoporosis in these patients.