Serum and urinary variables of bone mineral metabolism were studied in 49 postmenopausal women with rheumatoid arthritis (RA). Results were compared to those in a sex, age and menopausal age matched control group. No patient took corticosteroids, or had any disease other than RA which might affect bone. Total serum calcium was low in patients with RA compared to controls (9.0 +/- 0.5 mg% vs 9.3 +/- 0.3 mg%, p less than 0.005), but was normal when corrected for albumin (9.5 +/- 0.6 mg% vs 9.3 +/- 0.4 mg%, NS). Serum phosphorus was significantly higher in patients with RA than in controls (3.6 +/- 0.3 mg% vs 3.3 +/- 0.4 mg%, p less than 0.001) as well as serum alkaline phosphatase activity (107.6 +/- 27.2 IU/l vs. 9.6 +/- 28.91 IU/l). Serum creatinine, vitamin D and parathyroid hormone levels were comparable in both groups. Urinary hydroxyproline and mucopolysaccharide excretions were higher in patients with RA than controls, both for fasting (respectively 0.089 +/- 0.028 vs 0.039 +/- 0.023, p less than 0.001 and 0.072 +/- 0.027 vs 0.047 +/- 0.019, p less than 0.001) and for 24 h values (50.3 +/- 17.9 mg vs 36.2 +/- 15.4 mg, p less than 0.001 and 54.6 +/- 26.0 mg vs 41.7 +/- 16.5 mg, p less than 0.05). Urinary calcium excretion was comparable in the 2 groups. Our findings of raised serum phosphorus and alkaline phosphatase activity, raised urinary hydroxyproline and mucopolysaccharides excretion suggest that in patients with RA there is a higher metabolic activity of bone. In none of the patients could any indication of osteomalacia or of parathyroid overactivity be found.