Malignant melanoma originating outside the skin is very rare, whereas primary malignant melanoma of the lung is extremely rare. 5-S-Cysteinyldopa (5-S-CD), a melanin metabolite, has been reported to be a prognostic marker for cutaneous malignant melanoma. This is the first report in the English language literature dealing with primary malignant melanoma of the lung using serum 5-S-Cysteinyldopa levels to monitor the effects of surgery and chemotherapy.