To clarify the development of binucleated cardiac myocytes (BCMs) in human hearts, lengths for BCMs and mononucleated cardiac myocytes (MCMs) were histologically measured and compared in 49 hearts weighing 13-730 g. The mean cell length was significantly longer in BCMs. The growth rate of cell length relative to the heart weight in BCMs and MCMs was almost identical and less than 0.333. The scattering of different lengths of BCMs and MCMs was much greater in adults, suggesting that each myocyte develops at a different rate. Short MCMs of 37.5 microns in hypertrophied hearts suggested that some BCMs might divide into two. In adult hearts, binucleation might arise, because BCMs with internuclear distance under 2.5 microns were found. The mean half of the sum of nuclear lengths of BCMs was shorter than the mean nuclear length of MCMs. They were largely constant in hearts weighing under 500 g but increased in larger hearts.