This paper discusses the relation between the lipophilic dimorphic yeast Pityrosporum ovale and seborrheic dermatitis. A review of studies concerning the microbiology in seborrheic dermatitis and immune reactions to P ovale are given. In our own studies with quantitative cultures, no significant difference was found in the number of P ovale in patients compared with controls, or between healthy and lesional skin in the patient group. IgG serum antibodies against P ovale cells estimated with indirect immunofluorescence did not show any difference between patients and controls, but a significant difference was found when a P ovale protein extract and ELISA were used. Immunological investigation on serum samples were done on 30 patients with seborrheic dermatitis. Defects were found in their T-cell function. The number of P ovale is of importance in those individuals who are susceptible to seborrheic dermatitis and the development of the disease depends on the way their immune system reacts to the antigens derived from P ovale.