A role of CD4(+) cells in the regulation of immune responses has steadily gained renewed recognition. The understanding of these T-regulatory (T-reg) cells in the generation of antitumor cytolytic T lymphocyte (CTL) response is therefore important. It has been shown that immunization with specific peptides, DNA, or tumor lysate-based vaccines can induce CTL responses in vivo. We have immunized melanoma patients with major histocompatibility complex (MHC) class I restricted peptide- or melanoma tumor lysate-loaded antigen-presenting cell (APC)-based vaccines and have monitored the generation of CTL responses and T-reg cell responses, if any. Using tetramer staining and limiting dilution analyses as monitors of CTL responses, we found significant increases in the number of antigen-specific CTL in circulation after vaccination with the MART-1(27-35) peptide (AAGIGILTV)-pulsed autologous APC, the MAGE-1(161-169) peptide (EADPTGHSY)-pulsed APC, or with autologous tumor lysate-pulsed APC. The antigen-specific CTL reached the peak expansion by day 7 and then declined to the prevaccine levels by day 28. The decline in the CTL response was associated by a concomitant expansion of CD4(+) CD25(+)T cells. Analysis of postvaccine peripheral blood lymphocytes (PBL) from patients showed an increased amount of interleukin (IL)-10 secretion on in vitro stimulation with IL-2 after successive vaccination. Triple color flow cytometric analyses revealed cytoplasmic IL-10 in the CD4(+)CD25(+) T-cell fraction and the number of CD4(+)CD25(+) IL-10(+) T cells were found to increase significantly in postvaccine PBL. These observations have implications in tumor antigen and APC/dendritic cell (DC)-based cancer vaccine strategies.