3-Oxoacyl-CoA thiolase occurs in 3 forms: 2 mitochondrial enzymes, 1 specific for acetoacetyl-CoA and 1 specific for 3-oxoacyl-CoA derivatives of higher chain length and 1 cytosolic enzyme, specific for acetoacetyl-CoA. It has been suggested that the mitochondrial acetoacetyl-CoA thiolase is involved in ketone body generation while the cytosolic enzyme participates in ketone body utilization. The present study concerns a patient with partial cytosolic acetoacetyl-CoA thiolase deficiency. The patient developed apparently normal during the first 2 months of life, but deteriorated rapidly during the following months. At the age of 12 months she showed hyperkinetic choreatic movements. At the age of 20 months she did not show voluntary movements and was deeply mentally retarded. Blood lactate and pyruvate were both increased, but the ratio was lower then normal (4.8 versus 11.7 for normal). She became extremely ketotic upon fasting and on a ketogenic diet. Thiolase activities of a needle liver biopsy were seperated by column chromatography on DEAE-cellulose according to Middleton (Biochem.J.132 (1973) 717). A low activity of the cytosolic acetoace-tyl-CoA thiolase activity (14 n moles/min x mg protein versus 39 n moles/min x mg protein for normal liver) was found. A further kinetic analysis of the residual activity revealed an increased Km for acetoacetyl (18 μM versus 8 μM for normal liver at infinitely low CoA) and an increased sensitivity for inhibition by CoA.