Consistency and reproducibility of serum prostate-specific antigen (PSA) measurement are essential in the application of this analyte to early detection or screening programs. In the present investigation, we sought to compare serum PSA levels determined by the IMx assay (Abbott Laboratories, North Chicago, IL) and the Tandem E (Hybritech Inc., San Diego, CA) to determine whether there were differences. Two hundred twenty-eight random sera from our archival bank were investigated. One hundred-eight specimens were in the Tandem range of 2.0-10.0 ng/ml, and prostatic histology was known based on either systematic sector needle biopsy or transurethral resection. PSA was measured with three different lost of the IMx and Tandem assays. Over the entire range, there was a good correlation (r2 = 0.985); however, in the more useful clinical range of 2.0-10.0 ng/ml, the correlation was reduced to 0.923; in the 2.0-6.0 ng/ml range, it was further reduced to 0.852. The slope for the entire range was 0.948; however, in the 2.0- to 10.0-ng/ml range, it was 0.894; in the 2.0- to 6.0-ng/ml range, the slope was 0.815. Using PSA cutoffs of 4.0, 5.0, and 6.0 ng/ml, significant decrease in abnormal PSA values in men with cancer was observed with the IMx compared with Tandem. These data suggest that the IMx and the Tandem PSA assays are not equivalent, and in most patients a lower value is realized with the IMx assay. This bias appears to be greater in men with prostate cancer. Clinicians must be aware which assay their patients are being tested with, and laboratory technicians should run internal standards to ensure lack of significant intralot variability and consistency over time.