In a retrospective study 161 hyperthyroid patients without treatment were divided into 74 with immunogenic hyperthyroidism (IMH) and 87 with non-immunogenic hyperthyroidism (NIMH). The frequency of complaints and the mean hormone concentrations were significantly higher in IMH and the median thyroid volume was significantly smaller. Diffusely reduced sonographic echoes were observed in only 50% of patients with IMH compared to 5% of those with NIMH. Homogeneous distribution of 99mTc in the thyroid was observed scintigraphically in 95% of patients with IMH and in only 3% of those with NIMH. Although the median of global thyroid uptake of 99mTc was significantly higher in IMH there was a broad overlap between the two groups. The mean hormone production is higher in IMH than in NIMH. In order to separate IMH and NIMH, several criteria have to be employed which differ concerning their diagnostic significance.