Gingival overgrowth, which is one of the major side-effects of this immunosuppressive agent, was studied in specific pathogen-free Fischer rats fed diet No. 2000 containing 56% sucrose. Marked macroscopic overgrowth was noted in mandibular gingiva of all the rats fed this diet containing cyclosporin A. The overgrowth was more severe in buccal than in lingual gingiva. Rats fed the cyclosporin-containing diet and infected with Streptococcus sobrinus 6715 had the most gingival overgrowth. Histopathological examination revealed that the bulk of the enlargement consisted of fibrous connective tissue without a marked increase in the number of fibroblasts or inflammatory cells.