In a prospective multicentric study, 924 untreated hyperthyroid patients were investigated, coming consecutively within one year into 17 thyroid centers of 6 European countries. With the aid of clinical information, evaluation of thyroid scan and centrally assayed thyroid hormones, thyroid antibodies, TSH-binding inhibiting immunoglobulins (TBII), and urinary iodine, different types of hyperthyroidism could be shown. Two types of hyperthyroidism could be defined directly: autonomous adenoma in cases of hot nodules in thyroid scan and Graves' disease, defined as hyperthyroidism with eye symptoms, and/or measurable TBII levels. The remainder, called "non-classifiable", included TBII negative Graves' patients, comprising of Hashitoxicosis, toxic nodular goiter, and other multifocal autonomies. 9.2% of the patients had an autonomous adenoma, 59.6% Graves' disease, and 31.2% unclassified hyperthyroidism. The main and significant difference between these types were mean age, goiter size, nodularity, and severity of the disease, being especially expressed in Graves' disease. Graves' patients had significantly increased T3/T4 ratios. Using as additional criteria diffuse regular uptake and/or increased T3/T4 ratios for immunogenic types of hyperthyroidism at least half of the 31.2% unclassified hyperthyroidism are probably Graves' disease. Forming two groups of iodine-deficient areas (IDA) and iodine-sufficient areas (ISA) according to the urinary iodine, it was possible to elucidate some characteristics independently of local factors. Autonomous adenoma was more frequent in IDA (10.1%) than in ISA (3.2%). Differences in iodine supply are reflected in the three types of hyperthyroidism by a significant higher prevalence of goiter, thyroid nodularity, lower thyroid hormone concentrations, and a higher rate of T3 toxicosis in IDA.(ABSTRACT TRUNCATED AT 250 WORDS)