OBJECTIVES: A disintegrin and metalloprotease (ADAM) 9 has been implicated in tumour progression of prostate cancer. We evaluated the expression of ADAM9 on protein and messenger RNA (mRNA) level in a larger cohort of prostate cancer cases following prostatectomy and correlated the findings with clinicopathological parameters including prostate-specific antigen (PSA) relapse times. METHODS: We immunostained 198 clinicopathologically characterised prostate cancer cases for ADAM9. For 25 additional cases, ADAM9 mRNA of microdissected tumour and normal tissue was analysed via quantitative reverse transcriptase-polymerase chain reaction. RESULTS: ADAM9 was significantly upregulated in prostate cancer compared with normal tissue on mRNA and protein level. ADAM9 protein expression was significantly associated with shortened PSA relapse-free survival in univariate and multivariate analyses, particularly in patients who had received prior androgen ablation. CONCLUSIONS: ADAM9 is overexpressed in prostate cancer cases and is an independent prognostic marker of PSA relapse-free survival following radical prostatectomy. Further studies are needed to verify its role as a predictive marker of response to androgen ablation.