Acanthosis Nigricans (AN) is a skin manifestation of diabetes mellitus. It is also a skin sign of other internal diseases including some hereditary diseases and internal cancer. It is commonly seen in those who are obese. Skin changes are characterised by dark, brownish black velvety thickening of the skin folds such as the armpits, upper back, neck and groins.

This skin disease may be associated with cancers in the gut and urine canal. AN often occurs simultaneously with the cancer and its course parallels that of the tumour. AN may be hereditary or associated with noncancerous diseases including diabetes mellitus, thyroid disease, hormone therapy and obesity.

In this condition, the skin folds in areas such as the armpits, the neck and the groins appear thickened, dark and velvety. The nipples, navel, elbows and knees may also be affected.