Myoadenylate deaminase deficiency (MADD) is the most common metabolic muscle disorder. Here we report the largest study to date of MADD in Spanish patients, including clinical, histological, and molecular data. Most of the patients presented with moderate clinical symptoms of exercise intolerance, including myalgia, fatigability and cramps. In 70% of the patients, serum creatine kinase (CK) was elevated. Muscle biopsy showed mild, nonspecific alterations with absent histochemical reaction for MAD. Eight cases ofMADD were coincidental with other associated diseases, and had more severe tissue alterations upon muscle biopsy. The mutation C34T in the MAD gene was present in a homozygous state in 26 of the 27 patients. One patient was a compound heterozygote for the C34T/G468T mutations. We conclude that MADD should be suspected in patients with exercise intolerance and with idiopathic hyperCKemia. Since symptoms may be subtle, we recommend routine histochemical analysis of MAD in all muscle biopsies, followed by molecular analysis in MAD-negative cases.