Upper respiratory tract (URT) viral infections may cause severe consequences during myeloablative bone marrow transplantation (BMT). We present a patient with parainfluenza virus (PIV) infection during the course of BMT. He remained relatively asymptomatic during the course of cytopenia, but presented with complete loss of voice and severe laryngitis a few days after engraftment, which is not usual for adult PIV infections. Seroconversion to PIV and marked increase in antibody titres was demonstrated, with complete lymphoid engraftment. Our case illustrated that the virulence of some URT viral infections depend on host immune factors, and may remain latent until graft versus host responses can be mounted.