The risk for opportunistic infections is correlated with low CD4+ T lymphocyte counts in patients with HIV. We performed a retrospective analysis in 54 patients with multiple myeloma undergoing high-dose melphalan chemotherapy + autologous peripheral blood stem cell transplantation to better define the value of routine control of CD4+ T lymphocyte counts in this important patient group. In 61% of our patients, CD4+ T lymphocyte counts after recovery from neutropenia were <200/microl and <100/microl in 24% (median = 181/microl). Overall survival, progression-free survival, response to antineoplastic therapy and frequency of post-transplant infections were not significantly different when patients with CD4+ T lymphocyte counts <200/microl and >200/microl were compared. However, overall survival was significantly shorter in the subgroup of 13 patients with very low CD4+ T lymphocyte counts (<100/microl) (P = 0.036). In 79.6% of all patients, at least one infection NCI-CTC grade II - IV developed within 100 days post-transplant. Opportunistic infections were rare. This analysis suggests that patients with CD4+ T lymphocyte counts < 100/microl may have a poorer prognosis.