Background: Multiple studies have linked fungal exposure to asthma, but the link to severe asthma is controversial. We studied the relationship between asthma severity and immediate type hypersensitivity to mold (fungal) and non-mold allergens in 181 asthmatic subjects. Methods: We recruited asthma patients aged 16 to 60 years at a University hospital and a nearby General Practice. Patients were categorized according to the lifetime number of hospital admissions for asthma (82 never admitted, 53 one admission, 46 multiple admissions). All subjects had allergy skin prick tests performed for 5 mold allergens (Aspergillus, Alternaria, Cladosporium, Penicillium and Candida) and 4 other common inhalant allergens (D. pteronyssinus, Grass Pollen, Cat and Dog). Results: Skin reactivity to all allergens was commonest in the group with multiple admissions. This trend was strongest for mold allergens and dog allergen and weakest for D. pteronyssinus. 76% of patients with multiple admissions had at least one positive mold skin test compared with 16%-19% of other asthma patients; (Chi squared p <0.0001). Multiple mold reactions were also much commoner in the group with multiple admissions (50% V 5% and 6%; p <0.0001). The number of asthma admissions was related to the number and size of positive mold skin allergy tests (Spearman Correlation Coefficient r = 0.60, p <0.0001) and less strongly correlated to the number and size of non-mold allergy tests (r = 0.34, p = 0.0005). Hospital admissions for asthma patients aged 16-40 were commonest during the mold spore season (July to October) whereas admissions of patients aged above 40 peaked in November-February (Chi Squared, p <0.02). Conclusion: These findings support previous suggestions that mold sensitization may be associated with severe asthma attacks requiring hospital admission. © 2005 O'Driscoll et al; licensee BioMed Central Ltd.