Contact sensitivity and immediate hypersensitivity to extracts from Pityrosporum ovale were studied in patients with atopic dermatitis (AD). In a chamber-scarification patch test, 75 (64%) of 118 patients with AD responded positively, compared with 1 (3%) of 35 healthy volunteers. However, no significant statistical correlations were found between contact sensitivity to P ovale in patients with AD and any of the following factors: age, sex, distribution of skin lesions, presence of pruriginous papules, history of infantile seborrheic dermatitis, or concomitance of other atopic diseases. Lymphocyte transformation test with P ovale antigen confirmed that those with positive patch test reactions showed significantly high stimulation indexes. The antigenic substances divided by gel filtration high-performance liquid chromatography were found in a fraction of components with molecular weights above 60 kd. In addition, 25 (71%) of 35 patients with AD showed a positive immediate response to P ovale extract in a prick test, whereas none of 11 healthy volunteers showed any response. Although the incidence of the positive immediate responses was similar to that in contact sensitivity, there was no clear correlation between the delayed and immediate hypersensitivity reactions. Based on these results, we think that P ovale plays a role as an allergen derived from the host environment in the exacerbation of the skin lesions of AD.