The aim of the study was to evaluate the effects of long-term green tea extract (GTE) supplementation on white blood cells (WBC) in CrossFit-trained and untrained men. Twenty-eight men were recruited for this study including CrossFit-trained (CF-TR) and untrained (CF-UNT) men. The study was carried out in two phases. Phase 1 involved non-supplemented untrained and trained men. Phase 2 involved the same groups but after 6-week supplementation with 2 GTE capsules once daily. Body weight, body composition and the number and percentage of peripheral blood leukocyte populations were assessed in each phase. The results revealed that the long-term green tea supplementation did not have a significant effect on body weight, body composition and the total white blood cell count of the study participants. However, the percentage of peripheral blood eosinophils increased while monocyte and neutrophil counts decreased. The changes in white blood cells were less pronounced in CrossFit-trained compared to control participants. Summing up, it should be noted that long-term GTE supplementation in CrossFit-trained men did not result in significant changes in body weight and composition and the total white blood cell count. However, the changes in leukocyte subsets were less pronounced in CrossFit-trained compared to untrained participants.