Serum growth hormone concentration was measured by radioimmunoassay in 31 cats with hypertrophic cardiomyopathy, 38 normal cats, and 35 cats with other cardiac disease. Cats with hypertrophic cardiomyopathy had a significantly increased serum growth hormone concentration when compared with normal cats and cats with other cardiac disease. The serum growth hormone concentration in cats with hypertrophic cardiomyopathy was less than that previously reported in cats with growth hormone secreting pituitary tumors. Pituitary tumors were not identified in eight of the cats with hypertrophic cardiomyopathy examined at necropsy. An increased serum growth hormone concentration may be measured in cats with hypertrophic cardiomyopathy but it is unclear if the increased serum growth hormone concentration is a cause or effect of hypertrophic cardiomyopathy.