The influence of underlying disease on documented infections has rarely been addressed in patients treated with high-dose chemotherapy (HDCT) and subsequent autologous peripheral blood stem cell transplantation (PBSCT). Because autografting has been used most frequently for malignant lymphomas and breast cancer, we analyzed in a retrospective study the data of 100 consecutive adult patients with either malignant lymphomas (group A, n = 50) or breast cancer (group B, n = 50) treated with HDCT at a single institution. The number of autografted CD34+ cells was not statistically different in either group. In this paper, we show for the first time that there is a significant difference in clinically or microbiologically documented infections in these groups of patients: documented infections occurred in 30% of malignant lymphoma patients but only in 4% of breast cancer patients (P=0.001). Of all isolated microorganisms, 78% were gram-positive. Because most of the documented infections were due to staphylococci, further studies should prospectively evaluate preventive measures to reduce the high incidence of these infections. This is especially important for lymphoma patients, who can be regarded as a high-risk group concerning gram-positive bacteremia.