A 43-year-old female with multiple congenital naevi presented with a 5-year history of right upper quadrant pain. Magnetic resonance imaging demonstrated a large epidural thoracic arachnoid cyst. This case highlights the need to consider that the management of patients with giant congenital melanocytic naevus is variable depending on the age of the patient, the location of lesions and the presence of complications such as neurocutaneous melanosis. The value of magnetic resonance scanning as a screening device is discussed.