BACKGROUND: Granulocyte macrophage colony-stimulating factor (GM-CSF) can be used as a potent stimulator for immune suppressed patients as defined by a decrease of human leukocyte antigen-D related expression on monocytes (mHLA- DR) after surgery. However, the exact role of GM-CSF on monocytic and T cell function is unclear. METHODS: In this retrospective randomized controlled trial (RCT) subgroup analysis, monocytic respectively T cell function and T cell subspecies of 20 immune suppressed (i.e. mHLA-DR levels below 10,000 monoclonal antibodies (mAb) per cell at the first day after surgery) patients after esophageal or pancreatic resection were analyzed. Each 10 patients received either GM-CSF (250 μg/m²/d) or placebo for a maximum of three consecutive days if mHLA-DR levels remained below 10,000 mAb per cell. mHLA-DR and further parameters of immune function were measured preoperatively (od) until day 5 after surgery (pod5). Statistical analyses were performed using nonparametric statistical procedures. RESULTS: In multivariate analysis, mHLA- DR significantly differed between the groups (p < 0.001). mHLA-DR was increased on pod2 (p < 0.001) and pod3 (p = 0.002) after GM-CSF application. Tumor necrosis factor-α (TNF-α) release of lipopolysaccharide (LPS) stimulated monocytes multivariately significantly differed between the groups (p < 0.008) and was increased in the GM-CSF group on pod2 (p < 0.001) and pod3 (p = 0.046). Th17/regulatory T (Treg) cell ratio was higher after GM-CSF treatment on pod2 (p = 0.041). No differences were seen in lymphocytes and T helper cell (Th)1/Th2 specific cytokine production after T cell stimulation with Concanavalin (Con) A between the groups. CONCLUSIONS: Postoperative application of GM-CSF significantly enhanced qualitative monocytic function by increased mHLA-DR and TNF-α release after LPS stimulation and apparently enhanced Th17/Treg ratio.