Abstract Objective: Hypertrophic obstructive cardiomyopathy has been proposed to be the result of gene mutations of contractile proteins. However, we have previously shown significant elevation of insulin-like growth factor-I (IGF-I) and transforming growth factor-β1 (TGF-β1) at the messenger RNA, protein, and receptor levels in patients with hypertrophic obstructive cardiomyopathy when compared with myocardium from patients without this disorder. We hypothesized that this growth factor overexpression is a regional phenomenon. To test this hypothesis, we compared levels of IGF-I and TGF-β1 in hypertrophic and nonhypertrophic myocardium within the same group of patients with hypertrophic obstructive cardiomyopathy. Methods: Two biopsy specimens were obtained from each patient undergoing septal myectomy for severely symptomatic hypertrophic obstructive cardiomyopathy, from hypertrophied septum and from nonhypertrophied myocardium (8 patients in total). Clinical data were prospectively recorded. Messenger RNA levels for growth factor were quantified by means of multiplex reverse transcriptase–polymerase chain reaction, expressed as a densitometric ratio of growth factor/glyceraldehyde-3-phosphate dehydrogenase. Protein levels were quantified by means of chemiluminescent slot blot analysis. Growth factor proteins were used to generate a standard curve. Results: IGF-I messenger RNA and protein levels in hypertrophic myocardium were 2.6 and 2.9 times greater, respectively, than in nonhypertrophic myocardium of the same patients (both P <.01). TGF-β1 messenger RNA and protein levels in the hypertrophic myocardium were 2.5 and 2.8 times greater, respectively, than the levels in the nonhypertrophied myocardium (both P <.01). There was a significant correlation between the IGF-I protein ratio (hypertrophic/nonhypertrophic myocardium) and the inducible left ventricular outflow tract gradients measured at cardiac catheterization (r = 0.77, P =.025). Conclusions: Myocardial overexpression of IGF-I and TGF-β1 is a regional phenomenon in patients with hypertrophic obstructive cardiomyopathy and is likely involved in the pathogenesis of the disorder.