Serum dimethadione (DMO)/trimethadione (TMO) ratios after oral administration of TMO have been investigated in 10 patients with normal livers, 8 patients with hepatoma and 8 patients with hepatoma and cirrhosis. Serum concentration ratios of DMO to TMO at 4 h after oral administration of TMO in patients with chronic liver disease were significantly decreased by 27% for those with hepatoma and 52% for those with hepatoma and cirrhosis. Serum DMO/TMO ratios at 4 h correlated well with liver function characteristics (total protein r = 0.741, plasma albumin r = 0.826, total bilirubin r = -0.725, cholinesterase r = 0.853) as well as with pharmacokinetic parameters (total body clearance r = 0.852, half-life r = -0.636) in both patients with normal livers and patients with chronic liver disease. This study suggests that serum DMO/TMO ratios in a blood sample obtained by a single collection after an oral administration of TMO might provide a clinically useful index of the hepatic drug-oxidizing capacity in an individual patient with chronic liver disease without determining the liver function characteristics or the pharmacokinetic parameters.