Little is known about the epidemiology of rotavirus infection in Turkey. The aim of the study was to determine the incidence and clinical significance of rotavirus gastroenteritis, in view of the potentially available prevention by rotavirus vaccination. The study also sought to determine possible risk factors for rotavirus gastroenteritis. Therefore, 920 children under five years of age with acute gastroenteritis admitted to three pediatric hospitals in Izmir were studied. Rotavirus was identified in 39.8% of the children. Most children with rotavirus gastroenteritis (80.7%) were younger than two years of age. Marked seasonality of rotavirus gastroenteritis was observed, with a peak incidence from January to March. A total of 91% of rotavirus strains that were typed were of serotypes G 1-4. There was no significant difference among rotavirus-positive and rotavirus-negative patients with regard to family income. Compared with children who were exclusively breast-fed, those who were not exclusively breast-fed were at a two-fold greater risk of rotavirus diarrhea. Rotavirus gastroenteritis was significantly more severe than non-rotavirus gastroenteritis; 69% of children with rotavirus infection had severe gastroenteritis (score > or = 11). In conclusion, rotavirus is the most common cause of severe gastroenteritis among children under five years of age in Izmir. A new potent rotavirus vaccine, when available, will provide effective protection against severe rotavirus infection. Promotion of breast-feeding would augment the impact of rotavirus vaccines in preventing severe childhood diarrhea.