BACKGROUND: Respiratory viruses are known to cocirculate but this has not been described in detail during an influenza pandemic. OBJECTIVES: To describe respiratory viruses, including co-infection and associated attributes such as age, sex or comorbidity, in patients presenting with influenza-like illness to a community sentinel network, during the pandemic A(H1N1)pdm09 in Ontario, Canada. METHODS: Respiratory samples and epidemiologic details were collected from 1018 patients with influenza-like illness as part of respiratory virus surveillance and a multiprovincial case-control study of influenza vaccine effectiveness. RESULTS: At least one virus was detected in 668 (65.6%) of 1018 samples; 512 (50.3%) had single infections and 156 (15.3%) co-infections. Of single infections, the most common viruses were influenza A in 304 (59.4%) samples of which 275 (90.5%) were influenza A(H1N1)pdm09, and enterovirus/rhinovirus in 149 (29.1%) samples. The most common co-infections were influenza A and respiratory syncytial virus B, and influenza A and enterovirus/rhinovirus. In multinomial logistic regression analyses adjusted for age, sex, comorbidity, and timeliness of sample collection, single infection was less often detected in the elderly and co-infection more often in patients <30 years of age. Co-infection, but not single infection, was more likely detected in patients who had a sample collected within 2 days of symptom onset as compared to 3-7 days. CONCLUSIONS: Respiratory viral co-infections are commonly detected when using molecular techniques. Early sample collection increases likelihood of detection of co-infection. Further studies are needed to better understand the clinical significance of viral co-infection.