Café au lait macule (CALM) is a well-defined, evenly pigmented brown macule or patch. The term refers to the characteristic homogeneous color of "coffee with milk" that may be light to dark brown depending on the patient's normal skin pigmentation.

Onset is usually evident in early childhood as a solitary lesion, although they may be present at birth. A single lesion is present in 10%-20% of the US population, and 1% of healthy young adults have up to 3 CALMs. Lesions are more common in individuals of higher phototype (3% of Hispanic and 18% of African American newborns have a CALM). CALMs are located anywhere on the body, although they typically appear on the trunk or lower extremities and rarely on the face. They increase proportionally in size as a child grows and may vary in size (1 to >20 cm), but are usually 2-5 cm in adults and are asymptomatic.

Multiple CALMs should alert the clinician to the possibility of an underlying systemic disease. Many disorders are associated with multiple CALMs (as listed in the Differential Diagnosis section).