A population-based case-control study of the association of diet and other factors and thyroid cancer was conducted between 1980 and 1987 on Oahu, Hawaii. Study participants included 51 men and 140 women with thyroid cancer, and 113 male and 328 female controls matched on age (+/- 5 years) and sex. A significant, positive monotonic dose-response relation of weight in late adulthood (5 years prior to interview) and the risk for thyroid cancer was found for men and women. A greater than five-fold increase in the risk for thyroid cancer among men, and more than a two-fold increase in risk among women, was found for subjects in the highest compared with the lowest quartile of weight in late adulthood. Height was significantly related to the risk for thyroid cancer among men, but not women. Among men, there was a significant dose-response relation of weight in early adulthood (20-29 years of age) and the odds ratios (ORs) for thyroid cancer, although the trend was not significant after adjustment for height. Among women, there was also a positive relation of adult weight gain and thyroid cancer, with an OR of 2.6 associated with more than a 14% increase in weight. The effects of relative weight and weight gain on thyroid cancer risk were stronger in post-menopausal women than in premenopausal women. There was a significant positive interaction between fertility drug use and early adult weight and the risk for thyroid cancer in women. Odds ratios were also significantly elevated for women above the median weight in early adulthood who experienced a miscarriage or stillbirth at first pregnancy. In summary, these data show an association of weight, particularly in late adulthood, and the risk for thyroid cancer in men and women, and further suggest a positive interaction between weight in young adulthood and fertility drug use on thyroid carcinogenesis in women.