Cyclosporin A (CyA) and azathioprine (Aza) were compared with respect to renal side effects in an open controlled, randomized study of patients with rheumatoid arthritis. Twelve patients were treated with CyA (mean dose 7.8 +/- 1.2 mg/kg/day) and 12 with azathioprine for 26 weeks. All patients also received prednisolone 5 mg/day. The patients had normal serum creatinine (less than 120 mumoles/liter) and protein-free urine before the trial. CyA increased serum creatinine in nine out of the 11 patients followed for 26 weeks, the mean increase was approximately 50%. Creatinine clearance was reduced by 31%. Mean arterial pressure (MAP) and serum potassium were significantly increased by CyA. Urinary beta 2-microglobulin excretion was significantly increased by CyA, in five of the patients more than ten times. Urinary kallikrein excretion was reduced by more than 50% and urinary albumin excretion was doubled. All these parameters remained normal and unchanged in the azathioprine group. CyA was withdrawn in seven patients after 26 weeks. Urinary beta 2-microglobulin was still increased by 85% nine months after CyA treatment. The other parameters were gradually normalized after three to nine months except for one patient who developed renal failure. Urinary beta 2-microglobulin excretion was a very sensitive parameter for renal tubular damage in this study.