Objective: Despite overall good clinical results, cardiac surgery in high risk patients, such as those with poor left ventricular function or heavily hypertrophied myocardium, is still challenging. This study was designed to assess the efficacy of warm blood cardioplegia (WBC) in these two subgroups of patients. Methods: Fifty-two patients, with an ejection fraction less than 50%, who underwent surgical revascularization, and 36 patients, with marked left ventricular hypertrophy (LVH), who were operated on for aortic valve replacement (AVR), were consecutively studied. All of them received continuous retrograde 'warm' blood cardioplegia. Results were assessed on clinical outcomes and compared with those predicted from a risk- stratifying index which has been previously validated in a large multicenter population-based study (Ontario score). Results: For cardiac revascularization, the rates of overall hospital mortality, Q-wave infarctions and inotropic use were respectively 5.8%, 9.6% and 21.1% comparing favorably with those of the Ontario Group. For aortic valve replacement, the incidence of hospital mortality and Q-wave infarction was 2.8%. Conclusions: By virtue of the study design, these data cannot conclusively establish the superiority of warm blood cardioplegia over other methods of myocardial protection. However, they support the safety of this technique, and suggest that these subgroups of high risk patients might represent the elective indication for aerobic arrest.