The presence of Pityrosporum ovale was investigated in four groups of infants age 1 to 24 months, 15 with infantile seborrheic dermatitis, 15 with infantile atopic dermatitis, 15 with other infantile dermatoses, and 15 healthy infants. Samples were taken from the scalp, face, presternal area, and inguinal area. Pityrosporum ovale was detected by smears and/or cultures in 73% of infants with seborrheic dermatitis, 33% with atopic dermatitis, 33% with other dermatoses, and 53% of healthy infants. The percentages of positive smears and/or cultures from four body sites in each patient group were 42% for seborrheic dermatitis, 20% for atopic dermatitis, 20% for other infantile dermatoses, and 23% for healthy infants. The majority of infants with positive cultures or positive direct examination for P. ovale were between 1 and 8 months of age. The organism was isolated in 28% of samples taken from the scalp, 32% from the face, 30% from the presternal area, and 15% from the inguinal area. Patients with infantile seborrheic dermatitis were treated with 2% topical ketoconazole cream for two weeks. Eleven of these children were clinically cleared and 13 became mycologically negative. Pityrosporum ovale was significantly more frequent in infants with seborrheic dermatitis than in those with atopic dermatitis, in other infantile dermatoses, or in healthy infants, both in the total number of infants with positive smears and/or cultures and in the number of positive samples per body area (P less than 0.05).