The Nail Psoriasis Severity Index (NAPSI) is a numeric, reproducible, objective, simple tool for evaluation of nail psoriasis. This scale is used to evaluate the severity of nail bed psoriasis and nail matrix psoriasis by area of involvement in the nail unit. The NAPSI will be useful during clinical trials for evaluating response to treatment of psoriatic nails. The scale is reproducible, and because there are few data points, statistical analysis is simplified.