The study of the levels of thyroid antibodies was conducted in 58 patients treated with lithium and in 40 subjects who received other psychotropic drugs and who were used as controls. The measure of the antithyroglobulin antibodies were made by passive hemagglutination and that of the antimicrosomal antibodies by indirect immunofluorescence. For the whole control group, the research of the antithyroglobulin antibodies was positive in 3 cases, i.e. 7.5%. For the 58 patients treated with lithium, this research was positive in 11 subjects, i.e. 19%: thus, lithium seems to cause immunological reactions in the thyroid. 5 lithium-treated patients developed a goiter. The antibody determination was positive in only 2 subjects and was negative in the others. The measure of antithyroid antibodies before lithium did not enable us to predict the emergence of thyroid complications. On the contrary, subjects treated with lithium presented signficantly high antibody levels without developing clinical thyroid manifestations.