BACKGROUND: Biologics have demonstrated high efficacy in achieving 'almost complete' skin clearance in patients with moderate to severe psoriasis. Nonetheless, achieving 'complete' skin clearance remains a treatment goal for some highly biologics-resistant patients, as residual lesions impact their quality of life. OBJECTIVE: The risk factors for failure to achieve a Psoriasis Area and Severity Index (PASI) 100 response in patients with good response to biologics remain unknown. METHODS: This retrospective study evaluated the risk factors by comparing patients who achieved complete skin clearance (PASI100) with those who achieved almost complete skin clearance (PASI90). A database of 131 psoriasis patients treated with biologics, who achieved a PASI90 or PASI100 response, was reviewed from a tertiary referral hospital in South Korea. The patients were classified into PASI90 and PASI100 groups according to their PASI response. RESULTS: The PASI100 group had a lower prevalence of smoking history (adjusted odds ratio [OR], 0.34; 95% confidence interval [CI], 0.14-0.85; =0.021) and psoriasis on the anterior lower legs at baseline (adjusted OR, 0.18; 95% CI, 0.03-0.99; =0.049) than patients in the PASI90 group. CONCLUSION: This study suggested that smoking history and psoriatic skin lesions on the anterior lower legs are considered as the risk factors for the failure to achieve a PASI100 response in psoriasis patients treated with biologics.