Benign prostatic hyperplasia (BPH) is a highly prevalent condition in aging men, which can be progressive and lead to acute urinary retention (AUR) and the need for surgery. It is commonly treated with alpha-blockers and 5alpha-reductase inhibitors (5ARIs), both of which improve the symptoms of BPH. Long-term treatment with 5ARIs can also reduce the risk of developing AUR and the need for surgery. The landmark Medical Therapy of Prostatic Symptoms (MTOPS) trial demonstrated that over 4 years the combination of the type 2-specific 5ARI, finasteride and the alpha-blocker doxazosin was more effective than either agent alone in reducing overall clinical progression. Since the initiation of MTOPS, it has been shown that patients with larger prostates and higher prostate-specific antigen (PSA) levels are at greater risk of BPH progression, and are therefore arguably more likely to benefit from combination therapy. The Combination of Avodart and Tamsulosin (CombAT) trial is a 4-year, global, multicenter, randomized, double-blind, parallel-group study designed to investigate the benefits of combination therapy with the dual 5ARI dutasteride and the alpha-blocker tamsulosin compared with each monotherapy in improving symptoms and long-term outcomes in men with moderate-to-severe symptoms of BPH and prostate enlargement. Symptoms and long-term outcomes (AUR and surgery) will be assessed as separate primary endpoints at 2 and 4 years, respectively. Eligible patients were at least 50 years old with prostate volume > or =30 cm(3) and PSA level > or =1.5 ng/mL. A total of 4838 subjects have been enrolled. This paper describes the rationale, design and baseline data of the CombAT study.