Thirty-eight patients suffering from Famlial Mediterranean Fever (FMF) and undergoing colchicine therapy for periods varying from one week to three years were examined cytogenetically. Preparations were derived from short-term lymphocyte cultures; mitotic rate, percent tetraploidy, and chromosome breakage rates were determined. Twenty-one patients were examined prior to treatment, 22 during treatment and 5 both before and during treatment and 5 both before and during treatment. No statistically significant differences were observed in the parameters studied between ten controls and the patient groups. An in vitro experiment indicated a direct correlation between increased colchicine concentration and mitotic rate. However, tetrapoloidy or chromosome damage showed no such association with colchicine concentration. Among the patient group, pregnancy occurred in four patients while under treatment; three pregnancies resulted in the birth of normal children while the fourth has not yet been completed. In one preganancy, cultured fetal amniotic fluid cells demonstrated no effect of colchicine on the cytogenetic parameters invetigated. These results indicate no untoward effects on long-term colchicine treatment in FMF with respect to fertility, teratogenicity and chromosomal damage.