Longitudinal evidence that indoor allergen exposure causes morbidity in sensitized individuals with asthma is scarce. We evaluated the association of allergen sensitization and home exposure to short- and long-term morbidity in 140 women with asthma and to asthma prevalence in 458 women from metropolitan Boston. Cockroach (Blattella germanica), dust mite (Dermatophagoides farinae), and cat (Felis domesticus) allergens in home dust samples, and specific immunoglobulin E antibodies were measured at outset, and doctor-diagnosed asthma and markers of asthma morbidity were ascertained by questionnaire during a 4-year follow-up. Cat- and cockroach-sensitive (immunoglobulin E immunocap [Cap] class > or = 1) women with asthma reported greater morbidity in the past year at the start, and during follow-up, if high levels of the relevant allergen were found. Women with asthma sensitized to cat allergen and with concentrations at 8 microg/g or greater were more likely to have used steroid (adjusted odds ratio [95% confidence interval] 2.7 [1.2-6.2]) and wheezed without a cold (odds ratio 6.8 [3.3- 14.0]) during follow-up. Those sensitized and exposed to cockroach (Bla g 1 or 2 > or = 2 U/g) were at least three times more likely to have used steroid and to have attended a hospital emergency room; the size of the effect upon steroid use was maintained, but the precision was reduced and the 95% confidence interval included one (p = 0.07), with adjustment for race and poverty. We conclude that cockroach and cat allergens may contribute to asthma morbidity in sensitized women.