AIM: To determine the reference interval for serum total prostate specific antigen in healthy males of age 40years and above in a local population in Lagos. METHOD: This study was carried out on 130 Nigerian men living in Lagos between the ages of 40 and 80 years who were apparently healthy. Blood was collected from each subject and estimated for serum total prostate specific antigen (PSA) using a solid phase two-site enzyme immunoassay kit. Reference interval was derived for PSA in the general population and for specific age groups using the non parametric percentile guidelines of the International Federation for Clinical Chemistry (IFCC). Body mass index was obtained using weight and height and was correlated with PSA. The derived reference interval was validated by using 63 men who had histologically confirmed benign prostatic hyperplasia or carcinoma of the prostate and serum PSA results before biopsy or prostatectomy. Receiver operating characteristic (ROC) curve was plotted and area under the curve estimated. RESULTS: The reference Interval for serum total PSA is 0-4.6 ng/ml. There was significant reduction in PSA reference interval for obese and overweight individuals compared with normal weight subjects, values being 0 – 2.5 ng/ml in obese subjects and 0- 4.2 ng/ml in overweight subjects. There was statistically significant positive correlation between serum total PSA and age of subjects (r = 0.24, p = 0.0065). CONCLUSION: The newly derived reference interval showed better specificity, positive predictive value and efficiency than the traditional cutoff of 4.0 ng/ml. There was significant reduction in the PSA reference interval for obese subjects compared to normal weight subjects.