74 patients diagnosed as suffering from benign prostatic hypertrophy (BPH) were treated with prazosin for 8 weeks. We look at the evolution of the symptomatology and the urodynamic and morphologic parameters during the treatment and they are compared with the basal ones. The irritative symptoms (nocturia, pollakiuria, imperiosity and dysuria), evaluated by means of a quantitative scale, undergo significant statistical improvement. The same occurred with the obstructive symptoms. Urodynamically, an increase took place in total vesical capacity, maximum, and mean flow, as well as a drop or reduction in non-inhibited contractions. 27 patients suffered adverse effects, which obliged 8 of them to abandon the treatment. Global efficacy was excellent in 20% and good in 52%, whilst it was fair and ineffective in 19 and 9%, respectively.