This article analyses time trends and geographical variations of thyroid cancer by histological type. Incidence data were provided by 8 French cancer registries over the period 1978-1997, with 3853 adult cases reported. To assess the effects of age, period, cohort and area on incidence, log-linear Poisson regression models were used. Thyroid cancer increased exponentially from the cohort born in 1925. This increase was essentially due to papillary cancer, which increased by 6.2% per year in men and 8.1% per year in women over the entire period (1978-1997). In women, the recent trends were significantly different between the studied geographical areas. The analysis shows that the increase in thyroid cancer, essentially of the papillary type, is not recent. It may be attributed to a possible screening effect or to an increase in the number of "incidentally" discovered cases linked to the use of modern diagnostic tools.