Stem cell (SC) therapy for erectile dysfunction (ED) has been investigated in 35 published studies, with one being a small-scale clinical trial. Out of these 35 studies, 19 are concerned with cavernous nerve (CN) injury-associated ED while 10 with diabetes mellitus- (DM-) associated ED. Adipose-derived SCs (ADSCs) were employed in 18 studies while bone marrow SCs (BMSCs) in 9. Transplantation of SCs was done mostly by intracavernous (IC) injection, as seen in 25 studies. Allogeneic and xenogeneic transplantations have increasingly been performed but their immune-incompatibility issues were rarely discussed. More recent studies also tend to use combinatory therapies by modifying or supplementing SCs with angiogenic or neurotrophic genes or proteins. All studies reported better erectile function with SC transplantation, and the majority also reported improved muscle, endothelium, and/or nerve in the erectile tissue. However, differentiation or engraftment of transplanted SCs has rarely been observed; thus, paracrine action is generally believed to be responsible for SC’s therapeutic effects. But still, few studies actually investigated and none proved paracrine action as a therapeutic mechanism. Thus, based exclusively on functional outcome data shown in preclinical studies, two clinical trials are currently recruiting patients for treatment with IC injection of ADSC and BMSC, respectively. 1. Introduction Erectile dysfunction (ED) is a term recommended by a panel of experts in 1992 to replace the term “impotence” [1]. These experts also defined ED as an inability of the male to attain and/or maintain penile erection sufficient for satisfactory sexual performance. Although not life threatening by itself, ED is a strong predictor of high-mortality diseases such as coronary artery disease and cardiovascular disease [2–5]. ED does directly and negatively impact the quality of life of the afflicted men and their spouse [6–11]. In a 1999 report the worldwide prevalence of ED was estimated to be 152 million men in 1995 and predicted to increase to 322 million men by 2025 [12]. While the majority of ED cases can be treated with currently available medications or devices, approximately 20% of the overall ED patient population remains unresponsive to treatment [13], and in certain patient populations, such as those having diabetes mellitus (DM) or having undergone radical prostatectomy (RP), the failure rates are even higher, at 40% [14–16]. Moreover, regardless of their therapeutic efficacy or inefficacy, all current treatment options treat only the symptoms, not
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