An outbreak of rotavirus infections among newborns at Karolinska Hospital, Stockholm, which has been going on for greater than 2 years has been followed with clinical and epidemiological investigations. About one third of the babies born in the hospital were infected at the age of 3 days. The clinical symptoms were mild, 8.8% of the rotavirus positive babies had loose stools compared to 1.9% of those who did not excrete the virus. An epidemiological survey in the neonatal intensive care unit suggested that rotavirus was introduced into the unit by babies admitted from the obstetric wards. The main reservoir of rotavirus was the babies and rotavirus was not found among staff or mothers. In the beginning hygienic measures seemed to be effective but after some weeks the colonization rate again increased. Electropherotyping of samples collected during different periods showed that one single rotavirus electropherotype belonging to the subgroup 1 of human rotavirus was found throughout the outbreak.