Prostate diseases are increasing at alarming rate especially with aging men in our society. Prostate specific antigen (PSA) is a serine glycoprotein secreted by prostate tissues. It is a test used to diagnose prostate disease because it is not seen in healthy persons such as prostate cancer which is usually high in them. The study was done to determine the impact of PSA level on haematological parameters of men in Owerri. A total of one hundred men aged 52-70 years were recruited for the study who came for PSA tests. They were grouped into two according the level of PSA as Group A (PSA<4.0 ng/ml) and Group B (PSA ≥ 4.0 ng/ml).Serum blood samples were used for the PSA assay and anticoagulated EDTA venous blood used for haematological tests.The PSA was assayed using ELISA method.The results were analysed with student t-test. The results showed significant increase (P<0.05) in haemoglobin, PCV, RBC, MCV, MCH, MCHC, lymphocyte and eosinophil of the Group A (12.9 ± 0.8 g/dl, 38.0 ± 3.2%, 4.14 ± 0.3 X 1012/L, 89.62 ± 10.1fl, 30.44 ± 2.1pg, 340.2 v 15.2 g/l, 53.8 ± 8.2%, 1.4 ± 0.1%) compared to Group B (9.7 ± 0.5 g/dl, 28.7 ± 2.1%, 3.3 ± 0.2 X 1012/L, 87.23 ± 7.2 fl, 29.4 ± 1.3pg, 337.0 ± 20.4g/l, 34.67 ± 5.6%, 1.0 ± 0.1%) and significant decrease (P < 0.05) in WBC, neutrophil and monocyte of Group A (5.06 ± 1.2 X 109/L, 43.8 ± 5.0 X 109/L, 1.0 ± 0.1 X 109/L) compared to Group B (12.23 ± 3.1 X 109/L, 63.0 ± 7.2%, 1.3 ± 0.2%). The study showed significant decrease in all the haematological parameters studied except total white cells, neutrophil and monocyte showing that the increased level of PSA has a suppressing effect on the haematological parameters. Prostate specific antigen is really a biomarker to monitor prostate disease and clinical prognosis. Haematological profile of men with prostate disease should be monitored to avoid anaemia and infections. They should eat foods that contain antioxidants such as vegetables to reduce the free radicals and reactive oxygen released as a result of oxidative stress by the body.