Exercise Tl scintigraphy (EX-Tl) provides a noninvasive means of identifying myocardial perfusion abnormalities in patients (pts) with hypertrophic cardiomyopathy (HCM). We have noted that some pts with HCM have a pattern of transient dilation of the left ventricle (LV) on the immediate post exercise images as compared with 3 hour redistribution images. We presumed that left ventricular dilation was caused by subendocardial hypoperfusion. So we studied transient dilation of the LV in 50 pts with HCM and 20 controls (C). Initial and delayed conventional short tomographic images were obtained after reconstruction of 30 projections acquired over 180 degrees. Thirty six radii every 10 degrees were generated from the center of the middle myocardial images of the short axis. An area surrounded by the thirty six points of maximal count on each radius was calculated in initial and delayed images. Transient Dilation Index (TDI) as an index of dilation was determined by dividing an area in initial image by an area in delayed image. TDI in pts with HCM was larger than that in C. Pts with HCM were classified into the two groups, Group A: TDI greater than 1.11 (mean + 2 SD in C), 24 pts, Group B: TDI greater than 1.11, 26 pts. Frequency of pts with history of chest pain in Group A was higher than that in Group B, and frequency of pts with positive exercise ECG in Group A was higher than that in Group B. End diastolic volume in Group B did not change 10 minutes after exercise by radionuclide ventriculography. In conclusion, transient dilation of the LV in pts with HCM by EX-Tl is in appearance, and may reflect subendocardial ischemia.