The risks of prostate cancer and colorectal carcinoma increase with age. So, colonoscopy and measurement of serum prostate specific antigen (PSA) may be performed during a short term in a given patient. We aimed to evaluate whether colonoscopy affects serum PSA levels and to evaluate the relationship between prostate volume and elevation in serum PSA levels after colonoscopy. This study included 44 consecutive male patients, who underwent colonoscopy. The mean age of the patients was 56.05 ± 9.27 years. The mean time required for colonoscopy was 30 min. Serum PSA levels were measured 48-72 hours before colonoscopy, immediately after performing laxative enema, and at 24-48^th hour, the 7^th day, and the 14^th day after colonoscopy in each patient. The serum PSA level was elevated after enema and at 24-48^th hour and 7^th day after colonoscopy from the baseline (p < 0.05), and declined to the baseline by 14^th day. When the cut off value of 20 cm³ for normal prostate volume was taken into account, the serum PSA levels were significantly higher at the 24-48^th hour and the 7^th day in patients with larger prostate volume (> 20 cm³) than those with normal prostate volume (p = 0.013 and p = 0.009). These results suggest that PSA is easily released by manipulations from the larger prostate. In conclusion, serum PSA levels were elevated during 7 days after colonoscopy. Before performing invasive procedures, patients with high serum PSA levels should be asked whether colonoscopy was performed prior to the measurement.