The efficacy of neoadjuvant therapy for rectal cancer has not been assessed. To evaluate proportional changes in peripheral blood immune cells in rectal cancer patients after neoadjuvant therapy and assess the relationship between tumor regression and regulatory T (Treg) cells. Rectal cancer patients who had received neoadjuvant therapy prior to surgery at Shanxi Cancer Hospital between January and September 2018 were enrolled in the study. Treg, CD4+ T, CD8+ T, NK, B cells, and CD4+/CD8+ ratio in peripheral blood before and after neoadjuvant therapy were measured by flow cytometry. Patients were divided into down-staging and control groups, depending on their responses to neoadjuvant therapy. A total of 108 patients were enrolled. The proportion of Treg cells was significantly lower after neoadjuvant therapy (P < 0.05) but no changes were seen in CD4+ T, CD8+ T, NK, B cells, or CD4+/CD8+ ratio (all P > 0.05). There were 76 patients in the down-staging and 32 in the control groups. There were no significant differences in clinical parameters between down-staging and control groups (all P > 0.05). There were no significant differences in immune cell proportions between the two groups prior to neoadjuvant therapy (all P > 0.05). Treg, CD4+ T, and B cells were all significantly lower in the down-staging group after neoadjuvant therapy than before (P < 0.05). CD4+/CD8+ ratios were lower (P < 0.05) while proportions of Treg and natural killer (NK) cells did not change after neoadjuvant therapy in the control group. Following neoadjuvant therapy, Treg and B cells were lower while CD4+ and CD4+/CD8+ ratios were higher in the down-staging group compared to the control group (P < 0.01). Treg cells may constitute a reference for judging the effect of neoadjuvant therapy in rectal cancer patients.