Objectives Coronary vasodilator reserve is reduced in hypertrophic cardiomyopathy and secondary left ventricular hypertrophy despite angiographically normal coronaries. The aim of the present study was to assess whether quantitative differences exist between these conditions. Methods Using positron emission tomography with H215O, myocardial blood flow was measured at baseline and following intravenous dipyridamole (0·56 mg. kg −1) in 12 hypertrophic cardiomyopathy patients (age 34 (11) years, mean (SD), all male), 16 secondary left ventricular hypertrophy patients (age 58 (20) years, PResults Baseline myocardial blood flow in the hypertrophic cardiomyopathy patients was 0·82 (0·23) ml. min−1 . g−1 vs 0·94 (0·14) ml. min−1 . g−1 in its matched control group, P=ns. For the secondary left ventricular hypertrophy patient group, baseline myocardial blood flow was 1·17 (0·40) ml . min−1 . g−1 vs 1·06 (0·28) ml . min−1 . g−1 for the secondary left ventricular hypertrophy matched control group, P=ns. Following dipyridamole, myocardial blood flow was 1·64 (0·44) ml . min−1 . g−1 in hypertrophic cardiomyopathy patients vs 3·50 (0·95) ml . min−1 . g−1for the hypertrophic cardiomyopathy matched control group, P=0·0001. For the left ventricular hypertrophy patients, post-dipyridamole myocardial blood flow was 2·27 (0·60)ml . min−1 . g−1 vs 2·94(1·29) ml . min−1 . g−1 for the left ventricular hypertrophy controls, P 0·06. Coronary vasodilator reserve (dipyridamole-myocardial blood flow/baseline-myocardial blood flow) was 2·05 (0·61) for hypertrophic cardiomyopathy patients vs 3·81 (0·98) for the hypertrophic cardiomyopathy controls (P=0 0001, patients vs controls) and 2·06 (0·62) for left ventricular hypertrophy patients vs 2·90 (1·38) for the left ventricular hypertrophy controls, PP=0·0002 and in the secondary left ventricular hypertrophy patients, the values were 2·13 (0·64) vs 2·89 (1·42) in the secondary left ventricular hypertrophy controls, PConclusions In both hypertrophic cardiomyopathy and secondary left ventricular hypertrophy, the computed coronary vasodilator reserve is impaired, even after correction for baseline cardiac work. However, the extent of the reduction is greater in the hypertrophic cardiomyopathy patients. In the blunting of vasodilator reserve of secondary left ventricular hypertrophy, the patients' greater hyperaemic response is partly offset by the higher baseline myocardial blood flow.