Inhaled steroids may control bronchial inflammation in asthmatics exposed to allergens. In this study we evaluated whether prophylactic budesonide would prevent relapse of asthma in children re-exposed to offending allergens at sea level, after a period of antigen avoidance at high altitude. Thirty children received either budesonide (200 micrograms b.i.d.) or placebo (double-blind). Following a 4-wk baseline period and 2 wk of treatment at high altitude, children were treated for 3 mo at sea level. Methacholine challenge and pulmonary function studies were performed before and after baseline period, after the 2 wk of treatment in the mountain environment, and at the end of treatment. ECP serum levels were evaluated after the baseline period and at the end of treatment. PEFR and symptoms were recorded in a diary card during the study. The increase in methacholine provocative dosage was greater, although not significant (p = 0.096), in the budesonide than in the placebo group after the treatment at high altitude and remained higher at the end of the treatment (p = 0.04). ECP levels increased in both the groups with no significant difference. Our results confirm that budesonide, in addition to its efficacy in treating pre-existent airway inflammation, is effective in preventing the increase of reactivity in asthmatic children re-exposed to allergens.