A total of 62,475 children <5 years old from a defined population of approximately 500,000 children and adults from slums in New Delhi, India, were assessed for 1 year by means of passive surveillance, to identify children who were hospitalized for diarrhea. The incidence of severe rotavirus diarrhea was estimated, and the G and P types of the infecting rotavirus strains were determined and were correlated with the clinical severity of diarrhea. Of 584 children who were hospitalized with diarrhea, 137 (23.5%) had rotavirus detected in stool specimens (incidence of rotavirus diarrhea-associated hospitalizations, 337 hospitalizations/100,000 children <5 years of age). Most cases of diarrhea (98%) occurred during the first 2 years of life, peaking at 9-11 months of age. Rotavirus-associated diarrhea occurred year-round but was predominant in winter. Among the strains that could be G-typed, G1 was the most common serotype, followed by G9 and G2; 10% of cases of diarrhea were due to mixed G-type infections. Common strains identified in the present surveillance study were P[8]G1, P[4]G2, P[8]G9, P[6]G1, P[6]G9, and P[6]G3. Children infected with G1 strains had a greater risk of developing more-severe cases of diarrhea than did children infected with other rotavirus strains (odds ratio, 2.95; 95% confidence interval, 1.3-6.67).