Abstract: Human antimouse heterophile antibodies (HAMA) are naturally occurring antibodies that can interfere with modern serum assays. We report a case of HAMA interference with a commonly used prostate-specific antigen (PSA) assay, leading to false elevation (2.17–2.46 ng/mL) after radical prostatectomy. Pre-operative PSA was 4.4 ng/mL, and final pathology was Gleason 3 + 3, pT2cNXMX. This markedly elevated postoperative PSA led to unnecessary imaging for metastasis and psychological distress to the patient. Direct measurement of HAMA in the patient’s serum yielded a value of 440 ng/mL (<74 ng/mL). An alternate PSA assay using goat detection antibody eliminated interference, with all values 0.05 ng/mL. When a patient’s PSA is inconsistent with the clinical scenario, one should consider immunological interference by HAMA in PSA assays.