Abstract The transient elevated plasma growth hormone (GH) levels that occur at a young age in giant breed dogs may have consequences in adult life. The aim of this study was to investigate whether excess juvenile GH has consequences for cardiac function and morphology. To simulate the naturally occurring juvenile hypersomatotropism in giant breed dogs, elevated plasma GH and insulin-like growth factor-I (IGF-I) concentrations were induced in six miniature poodles (GH dogs) by daily administration of supraphysiological doses of GH starting at 12 weeks of age. Eight miniature poodles of the same age that received vehicle only served as controls. Cardiac anatomy and function were evaluated by echocardiography. After euthanasia at 21 weeks of age, the hearts were examined for weight, myocyte dimensions and collagen fraction. The hearts of the GH dogs had larger atria (+22%), a thicker left ventricular wall (+21%), greater weight (+84%), and their cardiomyocytes were 15% longer, 25% thicker, and 92% greater in volume than those of control dogs. The mean collagen fraction was also higher in the GH dogs (5.6%) than in the controls (3.1%). In conclusion, excess GH in juvenile miniature poodles resulted in myocardial hypertrophy and increased collagen content. These findings are consistent with observations in acromegalic human patients and in rats treated with GH.