The incidence of thyroid cancer in Norway increased about 2-fold for both sexes during the period 1955 to 1989 (4691 cases). In the last 5-year period, however, a decline was observed, especially among females. The ratio between age-adjusted incidence rates in Northern Norway compared with Southern Norway was 1.6 for females and 1.5 for males, and the incidence pattern during the period was similar in the 2 regions. The analysis of age-cohort-period models showed a strong cohort effect in both sexes. The reason for this is not clear, although radiation treatment during childhood and dietary habits may possibly be involved. A weaker but statistically significant and transient period effect was also present, giving relatively higher incidence rates, especially in the 1970s. Although this result may be consistent with an influence of radioactive fallout in the northern area, more detailed studies are needed.