Heterophile antibodies can cause falsely elevated levels of prostate-specific antigen (PSA), which is illustrated in this case report with two patient cases. In the first case, the falsely elevated PSA resulted in thorough and unnecessary examinations with multiple transrectal biopsies causing psychological distress and a risk of infection. In the second case, a patient was diagnosed with prostate cancer, and falsely rising levels of PSA possibly resulted in prolonged treatment with medical castration. These cases underline the importance of suspecting interference, when clinical findings and PSA levels do not match.