OBJECTIVES/HYPOTHESIS: Postviral olfactory dysfunction (PVOD) develops after a common cold, but little is known about the viral pathogen inducing olfactory dysfunction. We hypothesized that human parainfluenza virus 3 (PIV3) may cause PVOD. We therefore assayed the nasal cavity mucosae of PVOD patients for the presence or persistence of PIV3. METHODS: We assessed 25 patients (5 men, 20 women), ranging in age from 31 to 85 (mean, 51) years, diagnosed with PVOD and 22 controls (18 men, 4 women) diagnosed with nasal septal deviation between July 2005 and August 2006. Inferior turbinate epithelial cells were collected using a Rhino-probe mucosal curette, and PIV3 was assayed by seminested reverse-transcription polymerase chain reaction. RESULTS: PVOD occurred most frequently between May and July. Hyposmia was observed in 60% of patients and anosmia in 40%. The most common clinical symptoms were rhinorrhea, sore throat, nasal obstruction, fever, myalgia, cough, and hoarseness. Patients usually visited the outpatient clinic within 3 months after the onset of olfactory dysfunction. Twenty-two of 25 (88.0%) epithelial samples from PVOD patients were positive for PIV3 compared with 2 of 22 (9.1%) epithelial samples from controls. CONCLUSIONS: The high detection rate of PIV3 in the turbinate epithelial cells of PVOD patients suggests that PIV3 may be the causative virus of PVOD.