BACKGROUND: Although viral infections are a major cause of exacerbations in patients with chronic airway diseases, their roles in triggering bronchiectasis exacerbations in adults remain unclear. Therefore, we prospectively investigated the incidence and clinical impacts of viral infection in adults with bronchiectasis exacerbations. METHODS: The study cohort of 119 adults with bronchiectasis was followed up prospectively for 12 months. Nasopharyngeal swabs and sputum samples were assayed for 16 respiratory viruses, using polymerase chain reaction assays. Symptoms, spirometry, quality of life, bacterial cultures, and inflammatory markers were assessed during steady-state bronchiectasis and exacerbations. RESULTS: A total of 100 exacerbations were captured from 58 patients during 1-year follow-up. Respiratory viruses were found more frequently in nasopharyngeal swabs and sputum during bronchiectasis exacerbations (49 of 100, 49.0%) than during steady state (11 of 58, 18.9%; P < .001). The most common viruses found in patients experiencing exacerbations were coronavirus (19 of 65, 39.2%), rhinovirus (16 of 65, 24.6%), and influenza A/B viruses (16 of 65, 24.6%). Virus-positive exacerbations were associated with a greater increase in markers of systemic and airway inflammation (serum IL-6 and tumor necrosis factor-alpha; sputum IL-1beta and tumor necrosis factor-alpha) compared with virus-negative exacerbations, but the differences in spirometric indexes, quality of life, and bacterial density were unremarkable. In receiver operating characteristics analysis, serum interferon-gamma-induced protein 10 yielded an area under curve of 0.67 (95% CI, 0.53-0.77; P = .018). Furthermore, a greater proportion of patients with virus-positive exacerbations received IV antibiotics. CONCLUSIONS: Prevalence of viral infections, detected by polymerase chain reaction assay, is higher in cases of bronchiectasis exacerbations than in steady-state bronchiectasis, suggesting that respiratory viruses play crucial roles in triggering bronchiectasis exacerbations. The potential mechanisms of virus-induced bronchiectasis exacerbations merit further investigations. TRIAL REGISTRY: ClinicalTrials.gov; No.: NCT01801657; www.clinicaltrials.gov.