The population-based incidence of rotavirus gastroenteritis in children <5 years of age in Valencia, Spain, over a 1-year period (December 1, 2003, to November 30, 2004) was determined.A total of 553 episodes of gastroenteritis in children <5 years of age (mean age, 22.8 +/- 14.5 months) were recorded (annual incidence of 138 per 1,000). A positive enzyme-linked immunoadsorbant assay result for rotavirus antigen was obtained in 15% of the samples. The incidence of rotavirus gastroenteritis was 15 per 1,000 children <5 years of age, being the highest incidence in children <or=12 months of age (31 per 1,000). Eighty percent of rotavirus gastroenteritis occurred during the winter months (January-March). Genotype G1 was detected in 86% of the samples and G4 in the other 14%.