We evaluated the usefulness of the total myocardial uptake ratio (TMUR) of 15-(p-[123I]iodophenyl)-3(R,S)-methyl-pentadecanoic acid (123I-BMIPP) for predicting cardiac complications in patients with mitochondrial encephalomyopathy or myotonic dystrophy. Six patients with mitochondrial encephalomyopathy, four with myotonic dystrophy, and 10 control subjects were studied. Quantitative assessment of 123I-BMIPP dynamic myocardial imaging was performed, and the TMUR of 123I-BMIPP was calculated according to the Ishii-MacIntyre method. Then, the TMUR was compared in the 10 patients and 10 healthy controls, and all patients were followed for 56.1+/-22.1 months to evaluate cardiac complications. TMUR in patients (2.69+/-0.64) was significantly (P =0.01) lower than that in controls (3.28+/-0.25). Three patients in whom the TMUR value was above 3.00 had no cardiac complications. On the other hand, all patients in whom TMUR was below 3.00 had some kind of cardiac complication during the follow-up period. Two patients showed progressive conduction abnormality and underwent pacemaker implantation, one patient had sick sinus syndrome and underwent pacemaker implantation, another patient showed non-sustained ventricular tachycardia and paroxysmal atrial fibrillation, and four of seven patients, including one with a pacemaker, showed an increased cardiothoracic ratio value over 50%. In conclusion, measurement of the TMUR by the Ishii-MacIntyre method is useful for evaluating the development of cardiac complications in patients with mitochondrial encephalomyopathy or myotonic dystrophy.