To investigate whether partial inhibition of DNA synthesis affects the growth of thyroid cells, human thyroid cells were cultured in the presence of aphidicolin for 24h. After removal of aphidicolin, the growth of thyroid cells was evaluated by [3H] thymidine uptake and analysis of cell cycle by flow cytometry. In Graves' thyroid cells, [3H] thymidine uptake and the percentage of cells in S+G2/M phase increased during 24 hours after washout of aphidicolin, whereas those were not augmented in normal thyroid cells. Thus, the acceleration of DNA synthesis occurred after the partial inhibition of it by aphidicolin in Graves' thyroid cells, but not in the normal thyroid cells. This phenomenon might, at least in part, explain the difference of growth regulation between Graves' and normal thyroid cells.