We treated two cases of oral polypoidal malignant melanoma （PMM） with marked exophytic growth. We noted changes in levels of serum 5-S-cysteinyldopa （5-S-CD）, a tumor marker. Case 1 was a 51-year-old man. A brownish polypoidal mass of 55×40×40mm was found in the right mandibular gingiva, and was identified by biopsy to be malignant melanoma. 5-S-CD decreased from 12.5nmol/l before surgery to 3.9nmol/l after surgery. The 5-S-CD 3 years after operation is within the reference value range, and currently the course is good with no recurrence. Case 2 was an 81-year-old woman. A black polypoidal mass of 50×40×25mm was found in the right maxillary gingiva, and was identified by biopsy to be malignant melanoma. 5-S-CD decreased from 26.9nmol/l before surgery to 4.3nmol/l after surgery. The oral primary PMM was considered to be rare in these two cases. In addition, serum 5-S-CD could be useful in the oral cavity region.