A multi-centre study was designed to evaluate the efficacy of hydroxyzine in the treatment of patients presenting a generalized anxiety disorder (GAD). One hundred and thirty three patients, suffering from a GAD (according to DSM III-R criteria with 6 months duration criteria), were enrolled in a randomised, double-blind, hydroxyzine (50 mg/day) versus placebo, over a 4-week trial period. By the end of the first week, the decrease of anxiety scores was significant for the hydroxyzine group, as compared to placebo (in respect of all rating criteria of anxiety). The statistical superiority for hydroxyzine continued to the end of the 4-weeks study period, and persisted at a further evaluation a week after abrupt discontinuation of active treatment. The tolerance evaluation showed that side effects were reported in 52% of hydroxyzine group versus 35% of placebo group. The most commun side effects were sleepiness (28% vs 14% with placebo), weight gain (12% vs 10%), dry mouth (14% vs 5%), loss of concentration (9% vs 8%) and insomnia (9% vs 6%). Sleepiness in the hydroxyzine group appeared during the first week and progressively disappeared later during treatment. We concluded that hydroxyzine at 50 mg/day produces a statistically and clinically significant anxiolytic effect, commencing during the first week of treatment and maintained throughout the 4-week period and after abrupt discontinuation without rebound of anxiety or withdrawal symptoms. The most commun side effect with hydroxyzine is transient sleepiness.