Acanthosis Nigricans is skin disease that associated with hyperinsulinemia. Clinical is velvety hyperpigmented plaques on neck, axilla, groin. If hyperinsulinemia is improved by treated with oral metformin and/ or diet control, acanthosis nigricans would be improved as well. Hyperpigmented plaques will be changed. We assess objective measurement by using spectroscopic and colorimetric analysis.

Inclusion Criteria:

1. Subjects must have an elevated fasting insulin level, suggesting they are in an insulin resistant state.
2. Subjects must carry a diagnosis of acanthosis nigricans, which will be verified by a Dermatologist before entry into the study. If necessary, a small 4mm punch biopsy may be taken to document dermatopathology consistent with acanthosis nigricans.
3. Subjects must be willing and able to undergo treatment with Metformin, including initial referral and follow up.
4. Agree to abide by the investigator's guidelines
5. Be able to understand the requirements of the study, the risks involved and are able to sign the informed consent form
6. Agree to follow and undergo all study-related procedures

Exclusion Criteria:

1. Subjects with Type 1 Diabetes are excluded because of their naturally insulin-deficient, rather than hyper-insulinemic, states.

2. Women who are lactating, pregnant, or planning to become pregnant.

3. Any reason the investigator feels the patient should not participate in the study.

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Patients with both hyperinsulinemia and acanthosis nigricans.