OBJECTIVE Apoptosis has been implicated in the pathophysiology of various forms of heart disease. Acute cellular rejection leads to morbidity after heart transplantation and invasive techniques are needed for its diagnosis. We investigated the presence of cardiomyocyte apoptosis in transplanted hearts, its progression, its relationship with rejection, and the possibility that serological markers of apoptosis can be used to detect rejection noninvasively. METHODS Overall, 130 endomyocardial biopsies obtained sequentially from 14 consecutive patients during the first 6 months following heart transplantation underwent histochemical analysis. The degree of acute rejection was determined, myocyte apoptosis was assessed using the TUNEL method, and caspase-3 activity was measured. In the first 10 patients, soluble Fas, tumor necrosis factor-alpha (TNFα) and interleukin 6 levels were determined in serum collected at biopsy. RESULTS Apoptotic cells were detected in 81.5% of biopsies. No significant correlation was found between the apoptotic index and either the degree of rejection or the time from transplantation; there was only a trend to higher values during prolonged episodes of rejection, which did not reach statistical significance. An inverse correlation was observed between the degree of rejection and the TNFα level (rs=-0.33; P=.003). There was no correlation with any other variable. CONCLUSIONS Cardiomyocyte loss due to apoptosis was observed in transplanted hearts, but no correlation was observed with either acute rejection or the time from transplantation. Our findings suggest there could be an inverse correlation between rejection and the serum TNFα level. No serum parameter evaluated was regarded as suitable for the noninvasive diagnosis of acute rejection.