BACKGROUND: Human rhinovirus (HRV) infections are associated with exacerbations of asthma, chronic obstructive pulmonary disease, and sinusitis. Nitric oxide (NO) might play an important role in host defense through its potent antiviral properties. Previous studies have shown that HRV infection in human subjects increased nasal epithelial expression of type 2 nitric oxide synthase (NOS2), an isoform of the enzyme that produces NO. OBJECTIVE: We sought to investigate whether increases in exhaled NO (eNO) would accompany the increased NOS2 expression and would be associated with clearance of the virus. METHODS: Six human subjects were infected with HRV-16 intranasally. eNO from nasal and lower airways was measured by means of direct measurement at multiple controlled flow rates. eNO was monitored at baseline (day 1) and on days 2 to 5, 8, 14, and 42 after infection. Nasal lavages were performed on days 1 to 5 and 8, and nasal scrapings were performed on days 1 to 4. NOS2 mRNA expression in nasal cells was measured by using quantitative real-time RT-PCR. Viral shedding in nasal lavage fluid was monitored by using real-time RT-PCR and bioassay. RESULTS: Peak HRV titers and symptom scores were correlated on day 3, and HRV persisted until day 5 (n=4) or day 8 (n=2). Infection was associated with transient but significant increases in lymphocytes and monocytes in nasal lavage fluid. Significant increases in both nasal and lower airway eNO concentrations accompanied HRV infection and were positively correlated. Increased nasal eNO concentrations on day 3 were associated with increased expression of NOS2 mRNA in nasal scrapings. Symptom scores on day 4 were inversely correlated with the increases in nasal eNO concentration. CONCLUSIONS: We conclude that increased production of NO occurs as part of the host response to HRV infection and speculate that NO plays a beneficial role in viral clearance.