Renal biopsies were performed in 30 psoriatics during long-term low-dose cyclosporin (CSA) therapy (range 2.5-6 mg/kg/day) of from six months to eight years. The study included pretreatment biopsies in 25 of the patients. After two years all biopsies shared features consistent with CSA nephropathy despite completely normal pretreatment morphology in 18 of the 25 patients. The severity of the findings, which consisted of arteriolar hyalinosis, focal interstitial fibrosis and sclerotic glomeruli, increased with length of therapy. Mild renal lesions were seen during the first two years. After four years all but one had arteriolar hyalinosis, with interstitial fibrosis pronounced in five and moderate in six of 11 patients. At the same time glomerular sclerosis had become significant. A decrease in glomerular filtration rate (GFR) correlated with the severity of the fibrosis. GFR studied in 14 patients six months to seven years after discontinuation of CSA was still significantly decreased in relation to baseline prior to therapy. The data from our study together with experiences from cardiac-transplanted patients indicate that patients with psoriasis, after two years therapy with CSA, should be rotated to other treatments or be followed carefully by GFR and sequential renal biopsies.