• A prospective study of hospital-acquired rotavirus infection was undertaken on an infant and young toddler floor to assess the incidence of infection and risk factors associated with nosocomial transmission. During the seven-month study period, gastroenteritis accounted for 60 of 663 admissions, 23 (38%) of which were due to rotavirus. In spite of enteric isolation of 21 (91%) of 23 patients with community-acquired cases, 36 infants developed nosocomial rotavirus infections. The attack rate of hospital-acquired infection was 12.8%, ranging each month from 2% to 21%. Approximately three fourths of both community-acquired cases (17/23) and nosocomial cases (27/36) occurred during the late winter and early spring. Prolonged stays in the hospital were associated with an increased attack rate of rotavirus Infection. The risk of nosocomial rotavirus infection was not significantly enhanced by room contact with a rotavirus-infected patient or by the sharing of staff. However, only 47% (17/36) of patients with nosocomial infections were appropriately isolated, despite symptoms of gastroenteritis in all cases. These findings indicated that hospital acquisition of rotavirus is common, and indicate that failure to isolate patients with nosocomial rotavirus infections could be an important factor in hospital transmission.