Hair loss (alopecia) occurs as a manifestation of numerous systemic diseases, but usually can be categorized into one of five general groups: telogen effluvium, anagen arrest, follicular destruction, hair miniaturization, and hair shaft defects. An excess of hair also can be evidence of internal disease, and there are two general categories of increased hair density: hypertrichosis and hirsutism. The basic categories of hair disease and the systemic conditions associated with them are discussed. The history, physical examination, and histopathologic data usually are sufficient to categorize the form of hair disorder and may provide a clue to the nature of the underlying systemic disease.