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. 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. 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. The PASI100 group had a lower prevalence of smoking history (adjusted odds ratio [OR], 0.34; 95% confidence interval [CI], 0.14-0.85; p=0.021) and psoriasis on the anterior lower legs at baseline (adjusted OR, 0.18; 95% CI, 0.03-0.99; p=0.049) than patients in the PASI90 group. 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.