Several limitations of this study should be noted. First, the study sample was not randomly selected from the general population, but instead was recruited from a single hospital and clinic-based setting, the Vanderbilt Children's Hospital. While Vanderbilt hospitalizations represent greater than 90% of Davidson County/Nashville infant hospitalizations, it represents a smaller proportion of emergency department visits (51%), and likely even fewer paediatric acute care visits. 2 The relatively low participation rate among eligible subjects is multifactorial and the result of the long-term nature of the study, the lack of study personnel to enrol all eligible subjects, as well as lower willingness among outpatients to extend their visit in order to participate in the study. While this impacts the demographics and exposures of the study population, and thus generalizibility of our study results, it should not impact the findings of the role of infant viral infection on the outcomes of interest. Next, while airway hyperreactivity is not assessed in making the diagnosis of childhood asthma, the identification of incident asthma cases will take into consideration the positive response to a validated written questionnaire that has been compared with bronchial provocation testing. 6, 8 While such an ascertainment strategy might result in the underdiagnosis of asthma, it is unlikely to result in false positive diagnoses during the sixth year of life. Finally, as with many studies, where all eligible participants were approached for participation, difficulty was encountered in follow up of those currently age-eligible for follow up. Strategies to address this include study personnel doing a significant number of follow-up visits at the subject's home, and shipping follow-up materials and requests to paediatricians of subjects who have moved from the region.