In the literature there is considerable variation in the frequency of allergy to the moulds Alternaria iridis and Cladosporium herbarum. In 1985 a 6-month frequency study was performed in a specialist clinic with a patient population similar to the average specialist and out-patient clinics in Denmark as regards sex, age, and diagnoses. Allergy was confirmed by use of a purified and standardized extract, and SPT, CPT and RAST were performed. Among the 292 first referrals nine had allergy to Alternaria, and two to Cladosporium, corresponding to a frequency of 3.1 (1.0-6.5)% and 0.7 (0.1-3.6)%, respectively, with 99% confidence limits. Most of the patients were multi-allergics suffering from rhinoconjunctivitis as well as asthma.