Recurrent rhabdomyolysis due to decreased glycolysis occurred during strenuous exercise in patients with lactate dehydrogenase-A-subunit (LDH-A; muscle) deficiency. Enzyme activities of LDH in the muscle were decreased less than 8% of the control value. The isozyme pattern revealed only one band of B4. The level of LDH-A mRNA was not decreased. The direct sequencing of the reverse transcription-polymerase chain reaction product that corresponds to exon 6 revealed a deletion of 20 nucleotides. Immunofluorescence staining showed the presence of LDH-A protein within the cytoplasm. These findings suggest that an incomplete LDH-A protein lacking the subunit contact subdomain could not assemble into a tetrameric structure that has an enzymatic activity.