Five members of a family with von Recklinghausen's disease (v.R.d.) were studied with respect to the occurrence of sister chromatid exchanges (SCE) and chromosomal aberrations. The frequency of gaps, breaks and other chromosomal aberrations in cells from the v.R.d. family did not differ from those in controls. In the controls, the frequency of aberrations increased three‐fold after treatment with mitomycin C, but in the v.R.d. family no response was observed. The frequency of spontaneous SCE was significantly lower in all but one of the members of the v.R.d. family than in the controls. Mitornycin C increased the rate of SCE in all the v.R.d. family members and also in the controls, the increase being small, however, in two members of the v.R.d. family. The low frequency of SCE may be specific to the disease.