The renal effects of low-dose cyclosporin A (CsA) treatment in severe psoriasis was investigated in 10 patients treated with a mean CsA dose of 3.23 (range 1.94-4.10) mg/kg/day for 12 months. The psoriasis area and severity index was reduced by 63-76%. Ambulatory GFR (iothalamate-125I), ERPF (hippuran-131I), RVR and MAP were examined at 3-months intervals. A control renal biopsy was performed shortly before treatment start and a second biopsy was taken after 12 months of therapy. GFR was slightly but significantly reduced after 6 and 9 months; after 12 months the decrease was not significant (121.0 +/- 7.6 versus 115.2 +/- 7.8 ml/min/1.73M2, P > 0.10). After 12 months serum creatinine increased from 82 +/- 4 to 94 +/- 7 mumol/litre (P < 0.05), while an insignificant increase of ERPF was seen and FF decreased from 0.29 +/- 0.01 to 0.26 +/- 0.01 (P < 0.05). MAP remained unchanged. GFR and serum creatinine correlated significantly within each 3-month interval. A slight de novo interstitial fibrosis was seen in the second biopsy in 4 of 10 patients receiving a mean CsA dose of 3.2-4.1 mg/kg/day. In three of these patients a concomitant rise in serum creatinine was seen. In conclusion, low-dose CsA was associated with reversible fall in GFR and potentially progressive structural changes not always accompanied by corresponding functional alterations. One should consider reducing the daily dose of CsA to 3.0 mg/kg bodyweight or less in CsA therapy up to 1 year.