The etiology of osteoporosis associated with rheumatoid arthritis (RA) is unknown. We studied the calcium and vitamin D metabolism in 143 women with RA (mean age 50.7 years). Albumin corrected serum calcium was normal. Serum alkaline phosphatase was increased in 29 percent of cases. Serum vitamin D levels were frequently very low. In 16 percent of the RA patients serum 25(OH)D concentration was below 12.5 nmol/L, which is arbitrarily considered as the limit of vitamin D deficiency osteomalacia. In the winter season 73 percent of the patients had serum 1,25(OH)2D levels below the seasonally adjusted normal range. The lowest values were found in patients with high disease activity. We suggest that there is a disturbance in vitamin D metabolism in RA. This might play a role in osteoporosis associated with RA.