Linear and whorled nevoid hypermelanosis is a sporadic disorder of asymmetric hyperpigmentation in a streaky or swirl-like pattern. It is also referred to as pigmentary mosaicism. The hyperpigmentation is present at birth or within the first few weeks of life and is not preceded by any other eruptions in the skin. The pigmentation may become more evident over the first few years of life, then stabilizes, and only rarely fades.

In the minority of cases, associated anomalies have been detected involving the central nervous and musculoskeletal systems as well as ocular and other abnormalities.

This disorder most likely represents a mosaic population of neuroectodermal cells, resulting in 2 distinct populations of skin cells having different colors.

Both the pigmentary and systemic findings are thought due to genetic mosaicism, with mutations in various genes leading to the variety of phenotypes observed. Mosaicism for c-KIT ligand (KITLG) in the hyperpigmented affected skin has been detected in one well-studied case.