Recent data indicate that fungi may contribute to increased severity of asthma. To determine the prevalence of allergy to 15 mold allergens among patients hospitalized because of exacerbation of asthma and to evaluate the relationship between the severity of the disease and allergy to particular molds. Skin prick tests with standard aeroallergens of airborne allergens, including grass, tree, Dermatophagoides pteronyssinus, Dermatophagoides farinae, feather, and cat and dog fur, and a panel of mold allergens, including Alternaria, Cladosporium, Aspergillus, Penicillium, Trichothecium, Chaetomium globosum, Epicoccum, Epidermophyton, Helminthosporium, Aureobasidium pullulans, Rhizopus nigricans, Fusarium, Mucor, Merulius lacrymans, and yeast mix, were performed in 105 asthmatic patients and 30 controls. Positive skin prick test results were found in 98% of asthmatic patients and 66% of controls. Sensitivity to A pullulans was significantly associated with more severe asthma (odds ratio, 1.4; 95% confidence interval, 1.09-1.75; P = .006). Sensitization to Helminthosporium was associated with an increased number of asthma exacerbations that required hospitalization (17% vs 38%; chi2 test P = .03). Sensitization to A pullulans is a risk factor for severe asthma. Sensitization to Helminthosporium may be related to asthma exacerbation that requires hospitalization.