Segmental hypomelanosis and hypermelanosis arranged in a checkerboard pattern are distinct naevi: flag-like hypomelanotic naevus and flag-like hypermelanotic naevus.

The categorization of congenital hypo- or hyperpigmented skin lesions following a segmental pattern has been a long-lasting matter of debate and have been reported under various and often incorrect terms. To reassess published hypomelanotic and hypermelanotic lesions that did not follow Blaschko lines nor a phylloid pattern of mosaicism, we carried out an extensive and critical review of the worldwide literature. Seventy-four retrieved cases consisted of lateralized hypomelanotic lesions arranged in a flag-like pattern or appearing as large patches of grossly oval or angulated shape and sharp, serrated margins. Sometimes lesions harboured maculopapular melanocytic naevi or cooccurred with other segmentally arranged naevi. A probably non-random association with extracutaneous anomalies was also reported on rare occasions. In 70 cases, lateralized hypermelanotic patches were arranged in a flag-like pattern that often appeared as large quadrangular patches. Sometimes lesions harboured Spitz naevi. Ten cases belonged to phacomatosis melanorosea, whereas several others were part of so far uncategorized cases of phacomatosis pigmentovascularis. Flag-like hypomelanosis is a distinct naevus type, for which the term 'flag-like hypomelanotic naevus' is suggested. Its cooccurrence with extracutaneous abnormalities might represent a specific syndrome. Flag-like hypermelanosis is a distinct naevus type, for which the term 'flag-like hypermelanotic naevus' is suggested. Its co-occurrence with naevus roseus defines phacomatosis melanorosea. Flag-like hypermelanotic naevus should be distinguished from the checkerboard-like areas of darker skin as observed in chimaeras.