BACKGROUND: assessment instruments of nail psoriasis have been published in literature as means to standardize the previous subjective assessments of authors. OBJECTIVES: Using Nail Psoriasis Severity Index for evaluation in patients with plaque psoriasis during treatment with acitretin. METHODS: Thirty volunteers with plaque psoriasis were selected for the study. Twenty patients (8 women and 12 men) completed the study. The initial dose of acitretin was 0.3 mg/kg/day for 30 days and was later raised to 0.5 mg/kg/day. Nail Psoriasis Severity Index was collected in the first evaluation, after 2 and 4 months. Nails of both hands were evaluated. RESULTS: nail lesions were present in all patients at first evaluation. The initial Nail Psoriasis Severity Index median was 20 and the final score 20.5 (2.5% of worsening at the end of the study). No statistically significant difference between the three evaluations was found (X2 = 0.8084, GL = 2, p = 0.6657). Seven patients worsened in the final score. Three patients improved 50% of the initial Nail Psoriasis Severity Index and only one had an improvement of 75%. Linear correlation showed a weak association between the improvement percentage in PASI and Nail Psoriasis Severity Index (r = 0.105, F = 2.12, p = 0.162). CONCLUSION: The method was easy and of rapid execution while potentially bringing information about changes in nail plate and matrix during treatment. The Nail Psoriasis Severity Index does not quantify the existing lesions and might not have the sensitivity to detect small changes.