Morbidity and mortality from childhood asthma have been increasing in all developed countries over the past three decades, including in the United States. Numerous theories have been advanced to explain this asthma epidemic, but no single theory has held up to careful scrutiny. Recent international studies have suggested a relatively strong causal relationship between increased risk of childhood asthma and exposure to antibiotics during childhood, especially during the first year of life. The increased asthma risk was seen whether antibiotics were used to treat respiratory or non-respiratory infections. While these previous studies are suggestive, there are significant methodologic concerns about each study. A major concern with most of the studies is their reliance on retrospective recall of antibiotic exposure data from parents years after the exposure. The study relied on prospective data from the Childhood Asthma Study.