Two symptomatic patients with heterozygous carnitine palmitoyltransferase II (CPT II) deficiency are reported. Patient 1, a 21-year-old female professional tennis player, suffered from exercise-induced attacks of muscle pain, burning sensations and proximal weakness. Patient 2, a 30-year-old male amateur marathon runner developed muscle cramps and rhabdomyolysis upon extensive exercise and insolation-induced fever. In both patients, the common p.S113L mutation was found in heterozygote state. No second mutation could be found upon sequencing of all the exons of CPT2 gene including exon–intron boundaries. Biochemically, residual CPT activity in muscle homogenate upon inhibition by malonyl-CoA and Triton-X-100 was intermediate between controls and patients with mutations on both alleles. Although CPT II deficiency is an autosomal recessive disorder, the reported patients indicate that heterozygotes might also have typical attacks of myalgia, pareses or rhabdomyolysis.