Adrian J. Lowe, Ph.D., M.P.H., from Umeå University in Sweden, and colleagues assessed the effect of pollen exposure in the first and last 12 weeks of pregnancy and the first 12 weeks of infancy for all children conceived by women residing in Stockholm between 1988 and 1995. Hospital admission data were used to assess respiratory conditions in the first year of life.The researchers found that, by age 12 months, 940 of 110,381 children had been hospitalized for asthma. There were marked seasonal variations and between-year differences in pollen levels. There was an increased risk of asthma hospitalization for those exposed to high levels of pollen in the last 12 weeks of pregnancy (adjusted odds ratio [aOR], 1.35 for highest quartile versus remaining infants). Among children of mothers who smoked heavily, exposure to high levels of pollen in the first three months of life was associated with a significantly reduced risk of asthma hospitalization (aOR, 0.76).