OBJECTIVES: We describe molecular investigations of a large hospital outbreak of parainfluenza virus type 3 (PIV3), in which 32 patients became infected. We outline infection control measures that successfully limited further spread of PIV3 in a Haemato-oncology unit. METHODS: Clinical retrospective review of infected haemato-oncology patients was undertaken. PIV3 haemagglutinin sequences from each case (n = 32) and local epidemiologically unlinked controls (n = 53) were compared to identify potential linkage. RESULTS: PIV3-infected patients presented with upper (n = 18) and lower (n = 11) respiratory tract infections, 3 showed pyrexia only and one was asymptomatic. All symptomatic patients received antibiotics; bacterial co-infection was confirmed in eleven patients. PIV3 infections were associated with lower mortality than documented previously; three of the PIV3-infected patients died (3/32; 9%). All deaths were associated with relapsed malignancies, and PIV3 was not believed to be the primary cause of death in any of these patients. Sequences from 27 cases clustered closely together, consistent with nosocomial infections from PIV3 circulating within the ward. Factors favouring transmission were high patient turnaround between the day treatment unit and in-patient ward, and limited isolation facilities for immunocompromised and infected patients, especially within the day treatment unit. New infections reduced to baseline levels three days after enhanced infection control interventions were introduced. CONCLUSIONS: Molecular epidemiological analysis provided evidence for nosocomial transmission of PIV3 infection that facilitated effective implementation of infection control measures. These were instrumental in restricting further spread of the virus among high-risk patients.