Exceptionally high incidence rates of thyroid cancer have been reported for Melanesian women in New Caledonia (South Pacific). To investigate the occurrence of thyroid cancer in that country and to clarify the role of goiter and hormonal factors in that disease in women, a countrywide population-based case-control study was conducted in 1993-1999. The study included 293 cases, identified through pathology registers and whose thyroid cancer was verified histologically, and 354 population controls. Thyroid cancer was associated with goiter, age at menarche, irregular menstruation, and hysterectomy. There was a dose-response trend with number of full-term pregnancies (p = 0.01), with an odds ratio of 2.2 (95% confidence interval: 1.1, 4.3) for women with eight or more pregnancies. Miscarriage, particularly as an outcome of the first pregnancy, was also indicated as a risk factor. The association between voluntary abortion and thyroid microcarcinoma could be explained by enhanced medical surveillance and improved cancer detection in women undergoing abortion. Oral contraceptives and hormone replacement therapy were unrelated to thyroid cancer. The very high birth rate among Melanesian women in New Caledonia, as well as late age at menarche, may explain, in part, their elevated incidence of thyroid cancer.