The aim of this study was to determine the occurrence of gingival overgrowth (GO) among renal transplant recipients and to analyze possible background factors causing GO among cyclosporine-treated (CsA) patients as compared with others receiving azathioprine (Aza). A total of 32 recipients, 22 treated with CsA and 10 with Aza (ages 22 to 68 years) participated. The examination included determination of renal function, whole blood CsA concentration, and periodontal status. The tooth surface data were analyzed in terms of the given background variables for GO in the whole patient sample by logistic regression analysis. The occurrence of GO was significantly higher in the CsA group as compared with the Aza group (P less than 0.03). The CsA responders had significantly more gingival units overgrown as compared with the Aza responders (P less than 0.005). There were no differences in age, sex, whole blood concentration of CsA, or duration of CsA medication between the responders and non-responders. Expressed as individual means, the gingival inflammation scores were significantly higher among the CsA responders than among the CsA non-responders (P less than 0.005). Analysis of the surface data showed that CsA had an independent major effect on GO. Furthermore, simultaneous gingival bleeding increased the risk of overgrowth (odds ratio = 2). The results show that CsA medication is a significant factor for GO and the risk of the occurrence of GO is further increased by simultaneous gingival inflammation.