We studied the serial change of 123I-BMIPP SPECT in the process of hypertrophic cardiomyopathy (HCM). In 16 patients with HCM, the imaging data were acquired 15 minutes after injection of 111 MBq 123I-BMIPP. The SPECT image was divided into 17 segments and the severity of the defect in each segment was scored visually using defect score, a 4-point grading system (score 0 = normal, score 1 = mildly decreased uptake, score 2 = moderately decreased uptake, score 3 = severely decreased uptake or defect). Each patient underwent this examination twice with interval of 25 +/- 14 months. In the first examination, we observed reduced uptake in 14 cases (88%), and often found it in septal portion of anterior or posterior wall. In the second examination, compared with the first study, total defect score (TDS: the summation of defect score of 17 segments) had increased in 7 cases (44%) of 16 cases, decreased in only 1 case (6%), and unchanged in 8 cases (50%). The regional defect score often increased in septal portion of anterior and posterior walls, and the high score area widespread toward the septum. There was no difference between TDS unchanged group and TDS worsened group in the point of background (age, family history of HCM), the interval of the two studies, and the findings of echocardiography (Dd, Ds, %F). In TDS worsened group (n = 7), clinically only 1 case became progressed stage. In others there were no progression in each clinical, echocardiographic, or 201Tl scintigraphic findings. This study revealed that myocardial fatty acid metabolic disorder often progresses during short interval in patients with HCM. In conclusion, the serial examination of 123I-BMIPP SPECT may contribute to an estimation of the stage or the prognosis in HCM.