Background and Purpose: CD44 is an important metastasis-suppressor gene in prostate cancer. Downregulation of the CD44 gene is attributed to transcription repression by methylation of CpG islands in the promoter region. The feasibility of CD44 promoter methylation measurement as a diagnostic tool was assessed in the serum of patients with cancer of the prostate (CAP). Materials and Methods: Seven serum samples of patients with CAP were investigated for CD44 promoter methylation by methylation-specific PCR. Three patients had proven metastatic disease, and four were free of metastases. Tissues from a variety of normal epithelia were assessed as well. Results: Methylation of the CD44 promoter was readily detectable in all serum samples, although no distinction could be made between patients with and those without metastatic disease on the basis of the signal intensity of methylation-specific PCR products. Remarkably, tissue specimens from different normal epithelia, especially those of the colon and rectum, repeatedly showed aberrant methylation of the promoter region of CD44. Conclusions: In the serum of CAP patients, assessment of the methylation status of CpG islands in the promoter region of the CD44 gene is feasible using methylation-specific PCR. However, because of physiologic promoter methylation in normal tissues, including the colorectal mucosa, assessment of methylation of tumor-derived DNA in the serum of cancer patients lacks tissue specificity and seems not to be applicable in clinical settings.