Seborrheic dermatitis is a clinical diagnosis based on the location and appearance of lesions. In infants, it may present as thick white or yellow greasy scales on the scalp; it is usually benign and resolves spontaneously. In adolescents and adults, seborrheic dermatitis typically presents as flaky, greasy, erythematous patches on the scalp, nasolabial folds, ears, eyebrows, anterior chest, or upper back. The differential diagnosis is lengthy, but the correct diagnosis can usually be made clinically by the characteristic distribution of lesions and varying course of the disease.

If the diagnosis is uncertain, a biopsy demonstrating parakeratosis in the epidermis, plugged follicular ostia, and spongiosis can confirm the presence of seborrheic dermatitis. The diagnosis can be challenging in patients with darker skin, but the same principles apply.