Serum lipoproteins, bile and kinetics and net steroid balance were studied in 22 diet-treated and 5 insulin-treated patients with noninsulin-dependent diabetes mellitus, in 6 patients with insulin-dependent diabetes mellitus and in 15 normoglycemic controls. All subjects were middle-aged and the patients were hyperglycemic. Some of the diet-treated patients suffered from obesity and/or dyslipoproteinaemia mainly characterized by elevated levels of triglycerides and cholesterol in VLDL (very low-density lipoproteins). The diet-treated patients had enhanced fractional turnover of both cholic acid and chenodeoxycholic acid but bile acid formation was within the control range in all groups of patients. As the most significant finding net steroid balance (total cholesterogenesis) was raised in the diet-treated patients. Bile acid kinetics and net steroid balance were normal in insulin-treated patients irrespective of type of diabetes.