We conducted a prospective study to determine the clinical picture and impact of respiratory syncytial virus (RSV) on hospitalization for acute bronchiolitis of pediatric patients less than 2 years of age belonging to two different ethnic groups in Southern Israel: Jews and Bedouins. All patients younger than 2 years of age hospitalized for bronchiolitis during a typical RSV season were enrolled. During the study period 120 patients with bronchiolitis were hospitalized, and 83 (69%) were RSV-positive. Their age ranged from 20 days to 9 months. Fifty-five percent of all patients with RSV bronchiolitis were < or = 3 months old and 92% were < or = 6 months old. Patients with RSV bronchiolitis represented 18% of all hospitalized infants < or = 9 months old and 35% of all hospitalizations for respiratory problems of infants < or = 9 months old. The yearly incidence of hospitalization for RSV bronchiolitis was 5.4/1000 live births for Jews and 18/1000 live births for Bedouins. The total number of hospitalization days calculated for 1000 births was 32.1 for the Jews and 86.7 for the Bedouins. RSV has a serious impact on infant morbidity in Southern Israel.