Molds have been linked epidemiologically to asthma as a key aeroallergen in several studies. Other allergens such as cockroach have been linked to asthma in New York City (NYC). To our knowledge, however, the pattern of mold hypersensitivity has never been examined systematically in the NYC area. Thus, we sought to determine the association between mold hypersensitivity and asthma in a large group of ambulatory patients evaluated for allergic disease for the years 1993 through 2001 at a single medical center. Serological testing for mold-specific immunoglobulin E (IgE) as well as IgE specific for other aeroallergens was performed and the associations between allergen-specific IgE and the presence of asthma were examined using bivariate and multivariate analysis. Factor analysis showed that three distinct groupings of aeroallergen-specific IgE existed within the panel of allergens used. Group 1 consisted of cat dander and dust mites (Dermatophagoides farinae). Group 2 consisted of tree, grass, and ragweed pollen. Group 3 consisted of the Deuteromycetes molds, Alternaria tenuis, Aspergillus fumigatus, and Cladosporium herbarum. Patients with asthma had a highly significant increase in the incidence of hypersensitivity to cat/dust mites and to the molds. Multivariate analysis showed that the presence of hypersensitivity to either A. tenuis or C. herbarum had a significant independent association with asthma after adjustment for cat/dust mite hypersensitivity and after adjustment for other clinical factors. On the other hand, pollen hypersensitivity was not associated independently with asthma. Mold hypersensitivity was strongly correlated with hypersensitivity to cat or dust mites in patients who did not have asthma but not in patients who did have asthma. In the NYC area, recent pollen and spore counts show that mold spores are measurable in at least 75% of the year. Thus it is conceivable that mold hypersensitivity plays a contributing and independent role in initiating or perpetuating the allergic response in patients with asthma in the New York area.