One-hundred and two patients with rheumatoid arthritis (RA) were studied. They were divided into three groups according to treatment with gold salts, penicillamine or glucocorticoids. Blood samples were drawn between November and January and four different metabolites of vitamin D (25(OH)D3, 24,25 (OH)2D3, 25,26 (OH)2D and 1,25 (OH)2D) were measured and compared to values from normal subjects. The mean serum concentrations of 25(OH)D3 in all three patient groups were significantly lower than those of the controls (p less than 0.01-0.001). The mean serum concentrations of 24,25 (OH)2D3 and 25,26 (OH)2D were not significantly different from the control values, whereas 1,25 (OH)2D concentrations were significantly lower in the penicillamine and steroid groups (p less than 0.05-0.01). When patients were stratified according to functional classes, we found a significant inverse relation between serum concentrations of 25(OH) D3, 24,25(OH)2D3, 25,26(OH)2D and the functional class, but not between 1,25(OH)2D and the functional class. We conclude that the decreased serum 25(OH)D3 concentration found in patients with RA is likely to be caused by decreased exposure to sunlight due to decreased activity, and thus is a result of the disease rather than a pathogenetic factor. Whether the small decrease in serum 1,25(OH)2D is of clinical significance and related to the development of osteoporosis in patients with RA is probably doubtful.