An epidemiological survey was made of rotavirus infection in a children's hospital by determining changes in rotaviral genome patterns by means of polyacrylamide gel electrophoresis. In 180 fecal samples collected between December 1978 and June 1981, 53 distinct patterns were obtained belonging to 13 different types. Two of these patterns dominated in endemic nosocomial infections of neonates until 1981, when one of them was replaced by a third pattern. Two epidemics of diarrhea in the neonatal wards were associated with an endemic and an exogenic rotavirus strain respectively. The latter was not established as an endemic strain. Three patterns were found exclusively in isolates from infants with nosocomial infection and one pattern was found with a doubled fifth segment, suggesting a double infection. The latter findings support the conjecture that there is reassortment of rotavirus genomic segments in vivo.