Sister chromatid exchange (SCE) analysis is the most sensitive method for assessing chromosome damage induced by chemical mutagens. We report the SCE of peripheral blood lymphocytes in children with primary nephrotic syndrome (NS) treated with chlorambucil. Group I consisted of 20 normal children, group 2 of 14 children with primary NS who had never received a cytotoxic drug and group III of 7 children with primary NS who had received chlorambucil, which was discontinued 6–36 months prior to the study. Group IV consisted of 4 nephrotic children who were receiving chlorambucil therapy during the study. There was no significant increase in SCE in group III compared with group I or group II (P0.05). A significant rise in SCE (P<0.05) was="" seen="" in="" all="" patients="" in="" group="">