Fifty relatives of 7 families with high prevalence of obesity were investigated and the possibility was shown that there were three forms of familial obesity--normoinsulinemic obesity, hyperinsulinemic obesity and diabetic obesity. In normoinsulinemic obesity, both glucose tolerance and plasma lipids were normal with a few exceptions whereas in hyperinsulinemic obesity, mild glucose intolerance and manifest hyperlipidemia, and in diabetic obesity, blunted insulinogenic index and more advanced glucose intolerance with slight hyperlipidemia existed.