The hypothesis that consumption of seafood increases the risk of thyroid cancer has been tested by means of a matched case-control study. Linking the file of the National Health Screening Service (NHSS) containing dietary information about 60,000 Norwegians with the 1955-89 thyroid-cancer file of the Cancer Registry, by means of the 11-digit person-number, resulted in 92 cases--each of whom was matched with five controls with regard to age, gender, and place of residence. Forty-eight cases had answered questions on diet before diagnosis; 44 did so after diagnosis. Exposure data on seafood and seafood-related vitamins were recovered from the NHSS files for all 552 subjects. Odds ratios (OR) were computed by means of conditional logistic regression analysis. Univariate analysis of the 48 sets in which the case had answered the dietary questionnaire prior to the thyroid cancer diagnosis, as well as of all 92 sets, indicate that regular users of cod-liver oil, fish liver, or fish sandwich-spread run a higher risk of thyroid cancer than irregular and nonusers, and people eating more fish dinners per week also run a higher risk of thyroid cancer. Stepwise regression analysis corroborates the study hypothesis by showing that these two seafood variables increase the risk of thyroid cancer significantly. On the other hand, the results of a simultaneous regression analysis of these two seafood variables and a dietary vitamin-D index-variable tend to reduce the tenability of the above-mentioned conclusion since none of the OR estimates (all greater than one) reached significance in this part of the statistical analysis.