This article is a commentary on a study published in the journal *Environmental Health Perspectives*, 2005;13:357–361. This study, published in 2005, is one of the few to evaluate the role of indoor humidity and parental atopy in the development of childhood asthma. It is based on a six-year prospective follow-up of a cohort of Finnish children born between 1984 and 1989 and recruited in 1991. The statistical analysis uses Poisson regression, with adjustments for confounding variables. The role of parental atopy and exposure to indoor humidity and mold has primarily been assessed in cross-sectional or case-control studies, raising issues of information bias and the chronology of events. In this study, the prospective follow-up of a cohort of children over a six-year period aims to assess the role of various indicators of mold exposure on the subsequent onset of asthma. The combined effect of genetic predisposition to asthma (history of asthma or allergic rhinitis in parents) and environmental exposure is also tested. This study highlights the role of exposure to a musty odor in increasing the risk of developing asthma in a cohort of children followed prospectively. While other authors have previously demonstrated the role of indoor humidity in the onset of asthma-like symptoms, this study does not provide specific information on the nature (frequency and types involved) of mold exposure responsible for an increased risk of asthma onset in children. (Excerpts from the article).