Congenital melanocytic naevi can give rise to secondary melanocytic tumours, such as proliferative nodules and malignant melanoma. The clinical and histological features of both lesions may be nearly identical, which makes an unequivocal diagnosis impossible. In particular, it is difficult to differentiate clearly between benign and malignant proliferation in infants with secondary melanocytic proliferation. Reports on melanocytic proliferation and malignant melanoma within the paediatric age-group are very rare. There is limited expert knowledge on this subject and little is known about prognosis and outcome. We report here a case of an infant with an unusual transformation of a congenital spindle cell naevus of the umbilical region, and discuss clinical, histological and genomic criteria.