The diagnosis of Alternaria allergy is difficult and tests in vivo and in vitro usually do not give results as satisfactory as those obtained with other allergens. We studied 33 patients with a cutaneous response of 2 mm2 or more to skin-prick tests for Alternaria (extract containing 104 micrograms/ml of Alt-1). Results were compared with those of RAST. Skin tests were positive in 75.8% of cases (the resulting papule was the same size as or larger than that elicited by 10 mg/ml histamine) and negative in 24.2% of cases. RAST was positive in 75% of cases and negative in 25%. There was no significant difference in the frequencies. RAST was positive four times more often in males than in females; when these figures were corrected for population data, the ratio was 2:1. The coefficient of correlation between the intensity of skin-test and RAST results was r = 0.51 (p < 0.05). These findings are evaluated and results are discussed.