Side effects of antithyroid treatment were retrospectively analysed in 1256 patients with hyperthyroidism. Overall rate of side effects was 14.3%. Skin reactions were the most frequent ones (5.6%), followed by arthropathies (1.6%). The incidence of agranulocytosis was 0.14%. Median duration of all side effects was 1.5 months. In half the cases the side effects were controllable so that treatment was continued, although at a changed dosage. The rate of cross-reaction between carbimazole and thiamazole, on the one hand, and propylthiouracil, on the other, was 13.8% and 15.2%, respectively. The side effects became apparent after a mean of one month's treatment, almost always (in 97.1%) within the first year of treatment. There was a significant dose dependence for an initial thiamazole dose of over 20 mg (relative side effect risk of 2.3), and for an initial dose of over 30 mg for carbimazole (relative side effect risk of 1.6). Although most side effects were not dangerous, in normal instances the lowest possible dosage should be administered to control hyperthyroid metabolism. Long-term treatment with low doses seem to be without problems.