The long-term effects of different immunosuppressive drugs and regimens on renal allograft histology are virtually unknown. Therefore, in order to investigate the long-term effects of triple drug treatment versus different combinations of two immunosuppressive drugs on allograft histology, a prospective, randomized trial was performed. One group received triple therapy consisting of low-dose cyclosporin (CyA), azathioprine (Aza), and methylprednisolone (MP), and three groups received combinations of two drugs, i.e., Aza plus CyA, Aza plus MP, and CyA plus MP. At 2 years, there were no significant differences with regard to graft (80%) or patient (87%) survival, or to graft function between the four groups. After 2 years, a protocol core biopsy was taken of all 102 patients having a functioning graft. Of these patients, 61 (60%) were still following the original, randomized treatment protocol; in the remaining cases, changes had occurred in the original protocol and so these cases were considered drop-outs in this study. Histological specimens were examined blindly by two independent observers. Most of the 34 histological variables examined showed no changes. Diffuse fibrosis was most frequent in the CyA plus MP group (70%) and significantly more severe than in the triple therapy group. Mesangial matrix increase in glomeruli was significantly less common in the triple therapy group (8%) than in any one of the double drug combination groups (47%). Two other changes in glomeruli--Bowman capsular thickening and global glomerular sclerosis--were also less frequent in the triple therapy group.(ABSTRACT TRUNCATED AT 250 WORDS)