Studies of 20 children suffering from Down’s syndrome and of their parents have shown that the presence of thyroid autoantibodies was significantly more frequent in these children and in their parents than in the controls. Thyroid activity appeared to be normal in all cases of Down’s syndrome except for one child with mild hypothyroidism. Satellite association was found to be more frequent in cases of 21-trisomy (420 cells) than among controls (440 cells), but the difference was not significant. Satellite association with satellite approximation was, however, significantly more frequent in 21-trisomy. ‘G’ chromosomes were associated more frequently than ‘D’ chromosomes. The presence of thyroid autoantibodies had no effect on satellite association, either in cases of 21-trisomy or in healthy