The authors examined the thrombocyte aggregation in 10 controls and 17 patients with the diagnosis of primary hypothyroidism before and after 2 months substitution treatment with levothyroxine. They recorded a significantly reduced intensity of the aggregation response in untreated patients as compared with controls after adrenaline (p < 0.01), ADP (p < 0.01) but not after ristocetin. Impaired thrombocyte aggregation was observed in 11 of 17 patients, i.e. in 65%. After treatment the thrombocytopathy improved in 7 of 11 patients (63%), in four it persisted. Except one female patient the thrombocytopathy improved in all patients with manifest hypothyroidism. In patients with the latent form of hypothyroidism probably an independent coincidence of elevated TSH levels and impaired thrombocyte function was involved. The authors did not detect any cases of acquired von Willebrand's disease. In the conclusion the authors mention that impaired thrombocyte aggregation is a frequent phenomenon after thyroxine treatment. It may be of clinical significance when combined with other changes of haemostasis or in conjunction with the use of some drugs.