Seborrheic dermatitis is a common skin condition of infancy of unknown etiology. The disorder is usually mild and responsive to therapy. In severe or atypical cases, a variety of other entities should be considered in the differential diagnosis. Atopic dermatitis, psoriasis, psoriasiform-id reaction, fungal infections, and irritant contact dermatitis can be differentiated on clinical grounds. Less common but more serious disorders, such as histiocytosis X, immunodeficiency disorders, and nutritional and metabolic diseases, may be considered in the child whose disease does not spontaneously resolve or respond satisfactorily to local therapy. Laboratory studies including skin biopsy may be helfpul in these cases. Seborrheic dermatitis is uncommon in children after infancy and before puberty. In this age group, scalp scaling is likely to be due to other causes, such as tinea capitis, atopic dermatitis, or psoriasis.