Giant congenital melanocytic nevus (giant CMN) represents a subset of congenital melanocytic nevi (CMNs) defined by a size greater than 40 cm and is associated with an increased risk of melanoma and neurocutaneous melanosis.

CMNs are benign nevi present at birth or within the first few weeks of life. CMNs are often flat and tan in color, initially resembling an irregular café au lait macule, but may change during the first few years of life and can vary tremendously in size. CMNs are not thought to be inherited and are caused by mutations in the NRAS oncogene after embryo fertilization. They are most commonly found on the trunk (~38%) or extremities (~48%) and less commonly on the head and neck (~14%). They can be flat, elevated, or both and are primarily brown but may have variation in pigmentation including black, gray, and blue. Approximately 3% of patients have multiple CMNs. Compared with acquired nevi, CMNs have an earlier onset and are often larger with a more heterogeneous morphology.

One of the principal categorizations of CMNs is by size based on the projected maximal diameter of the CMN in adulthood. Different size cutoffs have been used with a recent proposed categorization including the categories of: small (< 1.5 cm), medium (M1: 1.5-10 cm, M2: > 10-20 cm), large (L1: > 20-30 cm, L2: > 30-40 cm), and giant (G1: > 40-60 cm, G2: > 60 cm). Most CMNs are less than 3-4 cm, with larger lesions being less common. Giant congenital nevi occur in approximately 1 in 20 000 newborns.

CMNs have a slight female predominance (3:2) and are more common among Black and Japanese individuals compared with Hispanic or White individuals.

CMNs have been reported to have an increased risk for transformation to melanoma. The risk of melanoma correlates with CMN size. The incident rate of melanoma arising in small or medium CMNs is reported to be less than 1%. In one meta-analysis, melanoma was estimated to develop in approximately 2% of large CMNs (> 20 cm), although some studies have reported a risk as high as 9% for giant CMNs. Melanomas were diagnosed at a mean age of 12 years (range: birth to 58 years old), with 82% cutaneous and 13% visceral. Most cutaneous melanomas were located on the trunk (68%), and most of the melanoma-associated CMNs exceeded 40 cm in diameter (74%), with 94% of patients also having satellite nevi.

Large CMNs overlying limbs can be associated with atrophy of the subcutaneous tissue. Large CMNs over the head or axial skeleton should alert the physician to assess for neurocutaneous involvement (developmental delays, seizures, hydrocephalus, increased intracranial pressure).

Neurocutaneous melanosis is when a CMN is associated with neuromelanosis, a congenital error in ectodermal morphogenesis resulting in melanocytic proliferation within the leptomeninges and brain parenchyma. This most commonly occurs in patients with giant CMNs. While not all patients are symptomatic, there can be diverse neurologic findings that typically present by age 2 years. The strongest risk factor for melanoma in patients with CMNs is the presence of central nervous system (CNS) abnormalities on MRI within the first 6 months of life. 

CMNs have been associated with many benign lesions as well as other syndromes. For further discussion, see congenital melanocytic nevus.

Pruritus is common in larger CMNs and can occur with or without secondary eczematization.