Phospholamban is an endogenous inhibitor of sarcoplasmic reticulum calcium ATPase and plays a prime role in cardiac contractility and relaxation. Phospholamban may be a candidate gene responsible for cardiomyopathy. We investigated genome sequence of phospholamban in patients with cardiomyopathy. PCR-based direct sequence was performed for the promoter region and the whole coding region of phospholamban in 87 hypertrophic, 10 dilated, and 2 restricted cardiomyopathic patients. We found a heterozygous single nucleotide transition from A to G at -77-bp upstream of the transcription start site in the phospholamban promoter region of one patient with familial hypertrophic cardiomyopathy. This nucleotide change was not found in 296 control subjects. Using neonatal rat cardiomyocytes, the mutation, -77A-->G, increased the phospholamban promoter activity. No nucleotide change in the phospholamban coding region was found in 99 patients with cardiomyopathy. We suspect that the mutation plays an important role in the development of hypertrophic cardiomyopathy.