Bone marrow (BM) and/or peripheral blood progenitor cells (PBPC) given after high-dose chemo-radiotherapy are commonly cryopreserved. Re-infusion of the thawed product can cause cardiovascular and other complications. We compared two groups of adult patients receiving autologous BM or PBPC transplant to assess the incidence of adverse events occurring during infusion. Fifty-one patients received BM, and 75 PBPC. The two groups were comparable in respect of age, total volume infused, quantity of dimethylsulfoxide (DMSO) and number of polymorphonuclear neutrophils. Patients receiving PBPC had a higher number of nucleated cells per kg of body weight; those in the BM group received a significantly greater quantity of red cells. Non-cardiovascular complications occurred in 19% and 8% of patients rescued by BM and PBPC respectively. The incidence of hypertension was 21% in the BM and 36% in the PBPC group. Asymptomatic hypotension was more frequent in PBPC patients (P<0.001). Bradyarrhythmia was noticed in two of 75 PBPC patients and in 14 of 51 BM patients (P<0.001). In the former group one patient had heart block; he died of renal failure 10 days later. Bradycardia and hemoglobinuria were more common in patients receiving BM where a higher concentration of red cells was present (P<0.001). Since bradyarrhythmias may be a life-threatening complication we advise continuous careful monitoring during infusion of thawed BM. The strong correlation between bradycardia and red blood cell contamination suggests the use of purified products with a very low red cell content.