Mitochondrial trifunctional protein (MTP) is a hetero‐octamer composed of four α‐ and four β‐subunits that catalyzes the final three steps of mitochondrial β‐oxidation of long chain fatty acids. HADHA and HADHB encode the α‐subunit and the β‐subunit of MTP, respectively. To date, only two cases with MTP deficiency have been reported to be associated with hypoparathyroidism and peripheral polyneuropathy. Here, we report on two siblings with autosomal recessive infantile onset hypoparathyroidism, peripheral polyneuropathy, and rhabdomyolysis. Sequence analysis of HADHA and HADHB in both siblings shows that they were homozygous for a mutation in exon 14 of HADHB (c.1175C>T, [p.A392V]) and the parents were heterozygous for the mutation. Biochemical analysis revealed that the patients had MTP deficiency. Structural analysis indicated that the A392V mutation identified in this study and the N389D mutation previously reported to be associated with hypoparathyroidism are both located near the active site of MTP and affect the conformation of the β‐subunit. Thus, the present patients are the second and third cases of MTP deficiency associated with missense HADHB mutation and infantile onset hypoparathyroidism. Since MTP deficiency is a treatable disease, MTP deficiency should be considered when patients have hypoparathyroidism as the initial presenting feature in infancy. © 2014 Wiley Periodicals, Inc.