Dutasteride is a new 5-alpha reductase inhibitor for the treatment of men with moderate to severe lower urinary tract symptoms secondary to benign prostatic hyperplasia. It has been available in the UK since March 2003. It is a competitive inhibitor of both type I and type II isoforms of the 5-alpha reductase enzyme that converts testosterone to the more potent androgen, dihydrotestosterone. Randomised controlled studies have shown dutasteride to be statistically more effective than placebo in reducing lower urinary tract symptoms and increasing maximum urinary flow rates. This is a consequence of a reduction in serum dihydrotestosterone and hormone dependent prostate volume. Dutasteride has also been shown to decrease the absolute risk of urinary retention and the need for prostate-related surgery when compared to placebo taken over a 24-month period. In this review article we discuss the pharmacology and clinical effects of dutasteride, a new dual-acting 5-alpha reductase inhibitor.