Objective To explore the changes and significance of lymphocyte subsets (CD3+, CD4+, CD8+ and CD19+ cells) and natural killer (NK) cells in type 2 diabetes mellitus (T2DM) patients with obstructive sleep apnea syndrome (OSAS). Methods Fifty patients with identified T2DM according to the WHO criteria for diagnosis of T2DM admitted to our hospital from June 2011 to June 2012 was enrolled in this study. Then, they were divided into T2DM group (n＝30) and T2DM+OSAS group (n＝20) according to the diagnosis and treatment guidelines of OSAS published in the Chinese Journal of Tuberculosis and Respiratory Disease in 2002. Another 25 healthy subjects without OSAS or T2DM, and 25 OSAS patients hospitalized in the same period were included as healthy control group and OSAS group, respectively. Minimal model of homeostasis was used to represent insulin resistance (HOMA-IR). Lymphocyte subsets in the peripheral blood and insulin levels in all four groups were detected by flow cytometry and radioimmunity. Multiple regression analysis was carried out to study the correlation of the percentages of lymphocyte subsets and NK cells with the apnea/hypopnea index (AHI), body mass index (BMI), fasting blood glucose (FBG), fasting insulin (FINS), and HOMA-IR. Results The patients of T2DM+OSAS group had significantly higher BMI, lower oxygen saturation (SpO2), and high incidence of cardio-cerebrovascular complications than those from normal control (P＜0.01). AHI, FBG, FINS and HOMA-IR were significantly higher, while the lowest SpO2 during the night was significantly lower in the OSAS, T2DM and T2DM+OSAS groups than in normal control (P＜0.05, P＜0.01). The percentages of CD3+, CD4+, CD19+ lymphocytes, NK cells and the ratio of CD4+/CD8+ were significantly lower in the above 3 groups than in normal control (P＜0.05), but there was no significant difference in the percentage of CD8+ cells(P＞0.05). Linear correlation analysis showed that the AHI was positively correlated with FINS and HOMA-IR. Multiple regression analysis indicated that there was significant correlation of HOMA-IR, ANH and FBG with the percentages of CD4+ T cells and NK cells. Conclusion Abnormal immunoregulation is involved in the incidence and development of T2DM and OSAS. Rational immunoregulatory therapy might be a new approach for T2DM patients with OSAS.