Fibroblast cell strains were obtained from skin biopsies taken from patients with adenomatosis of the colon and rectum (ACR), and their relatives. A total of 50 different fibroblast strains were tested for their frequencies of sister chromatid exchange (SCE) in vitro. These strains included nine from patients with the Gardner syndrome, 21 from patients with non‐Gardner ACR, and 20 cell strains from healthy relatives who were not at an increased risk for ACR. In 23 strains, the SCE frequencies after in vitro exposure to N‐methyl‐N'‐nitro‐N‐nitroso‐guanidine (MNNG) were also determined. Both with and without MNNG induction, SCE values in the Gardner strains were found to be significantly higher than in the control strains (p<0.02 and p<0.03, respectively). Non‐Gardner ACR strains differed only slightly from controls, thus making the difference between the control group and the pooled Gardner + non‐Gardner ACR group not significant. In all groups, there was a significant increase in SCE after MNNG exposure, and those strains which had low SCE values spontaneously, also tended to have relatively moderate SCE values after MNNG induction. There was no significant difference between the ratios of SCE values with and without MNNG exposure in the