Treatment of neutropenia with corticosteroids and hemopoietic growth factors may cause alterations in the immune system. This study investigated and compared the effects of various regimens (HDMP, G-CSF, and G-CSF + HDMP) on total lymphocyte counts and subtypes in neutropenic patients with acute lymphoblastic leukemia. Lymphocyte subtypes were studied just prior to treatment and 1 week later. In the HDMP group, total lymphocyte counts and the numbers of CD3+, CD4+, CD8+, and CD19+ lymphocytes and monocytes increased, while the helper/suppressor lymphocyte ratio decreased. No significant changes were seen in lymphocyte subtypes in the G-CSF and G-CSF + HDMP groups, apart from an increase in total lymphocyte numbers. In the control group there was an increase in the number of CD3+ and CD8+ lymphocytes and a decrease in the helper/suppressor lymphocyte ratio. These results suggest that HDMP has an inducing, rather than a suppressive effect on all lymphocyte subtypes studied.