This study was undertaken on 181 patients treated with bone marrow transplantation and in therapy with cyclosporin-A. 22 of the 181 (12.5%) exhibited a marked gingival overgrowth that covered more than 1/3 of crown length. The development of gingival overgrowth appeared in a range of 8 to 365 days, with a mean of 75 days, from the beginning of the immunosuppressive treatment with the drug. During the course of treatment, after a time period ranging from 30 to 365 days (mean 180 days), in 50% of the patients there was a total regression of the gingival pathology and in 36.4% a partial regression. It has not been possible to correlate the occurrence of gingival overgrowth with the presence of plaque or calculus and with the cumulative dosage of the drug. It is possible that a reduction in the daily drug dosage below a certain threshold may produce a partial or complete regression of the gingival pathology.