The frequency of sister chromatid exchange (SCE) was measured in peripheral blood lymphocytes from control, healthy subjects and immunosuppressed recipients of cadaveric donor kidneys with and without skin cancer. The mean SCE frequency in 43 control subjects was 9.2 per cell (range 5.4-12.3). In 30 transplant recipients with no history or evidence of cancer the mean SCE rate was 10.3 per cell (range 5.8-24.5); four (13%) of these patients had a mean SCE frequency outside the control range. In 7 transplant recipients with skin cancer, the mean SCE frequency was 14.3 per cell (range 9.1-19.9). This was significantly (P less than 0.01) higher than the mean value of control subjects. The mean SCE frequencies in 3 of these 7 patients fell within the control range and in 4 of these patients was above the control range. These results suggest that some immunosuppressed kidney transplant recipients are liable to chromosomal damage.