The frequency of several kinds of cancer among relatives of 200 patients with ovarian cancer was analysed. The annual population incidence was used as a control. Nonrandom familial clustering of ovarian cancer (p less than 0.01) was observed. The frequency of breast cancer in women and that of eosophagal cancer in man was higher than the expected value (p less than 0.05). The risk of ovarian cancer occurrence amounted 9% in women of the first degree of relationship, whereas cumulative risk in a population only reached 1.57% to the age of 90. The patterns of distribution of patients in the pedigrees satisfied the requirements of the multifactorial model. Heritability coefficient was 54.12 +/- 2.49%. Thus, women of the first degree of relationship compose the high risk group.