This paper presents evidence from Iceland which indicates that papillary thyroid carcinoma occurs in certain families more often than expected. Thyroid carcinoma was also seen to coexist with some other cancer types more often than expected. We studied all families (n = 373) with papillary thyroid carcinoma diagnosed between 1955 and 1984 in Iceland. Familial papillary carcinoma occurred in 3.8% of these families. This frequency was higher than expected but not significantly increased. Second primaries in women, and especially the incidence of kidney and breast cancer, were significantly increased. Cancer of the kidney and CNS tumours were significantly increased in propositi when both sexes were taken together. No increase in the incidence of other malignancies was observed in first degree relatives of patients with papillary thyroid carcinoma.