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Congenital malignant melanoma within a pre-existing large congenital melanocytic naevus (CMN) is exceedingly rare. Its incidence is difficult to determine due to the small number of reported cases and because of problems associated with diagnosis. Some benign nodular proliferations (called proliferative nodules) arising in CMN, while rare, are significantly more common and can mimic malignant melanoma clinically or histologically. There are no reported cases of congenital melanoma or benign proliferative nodules in CMN in patients who also had eruptive disseminated Spitz naevi. We describe a girl who was noted to have a dark-brown plaque with several large erythematous nodules affecting the scalp at delivery, in addition to multiple erythematous dome-shaped papules that developed in a disseminated manner over several months, beginning at 10 days of age. It was difficult, not only clinically but also histologically, to determine the benign or malignant nature of all of these lesions. As primary cutaneous melanoma, atypical proliferative nodules in CMN, bland CMN or CMN with foci of increased cellularity and Spitz naevi show clear differences in the genetic aberration patterns, comparative genomic hybridization (CGH) could be a diagnostic help in ambiguous cases such as this. CGH performed on this patient showed multiple DNA copy number changes in the most atypical nodule, but such alterations could not be found in the remainder of the lesions. CGH showed differences between the nodular lesions that occurred in the CMN and helped us in supporting the diagnosis of this unique case of benign proliferative nodules and a possible congenital melanoma arising in a large CMN, associated with multiple widespread eruptive Spitz naevi.

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