Deficiency of carnitine palmitoyltransferase type II (CPT II) is a clinically heterogeneous autosomal recessive disorder of lipid metabolism. The most common mutation in the CPT II gene is the S113L mutation, which substitutes leucine for serine at amino acid position 113. We studied an inbred family with three affected cousins with CPT II deficiency and found the S113L mutation to be present in a homozygous state in all three patients. Pedigree analysis traced the S113L mutation back to one common ancestor. Although the patients in this family have an identical genotype at the CPT II locus, their clinical picture ranges from asymptomatic to lethal.