Background: Erythematotelangiectatic rosacea (ETR) is a major subtype of rosacea. Although there are numerous severity assessment methods for each single clinical sign of ETR, there are few scale assessment methods for all signs of ETR. Objectives: To design a new severity assessment scale for patients with ETR and to evaluate the ability of the tool to help determine patient management. Methods: We designed the novel tool to assess the severity of ETR, which we named CAT (clinician erythema assessment [CEA], area and telangiectasia). Total CAT score is determined by CEA (0 to 4 points) × Area (1 to 3 points) + Telangiectasia (0 to 2 points), of which the range is 0 to 14 points. Six trained clinicians measured clinical signs of ETR (erythema, area, and telangiectasia) and CAT score (the total sum of those signs) in 80 patients with ETR subjects twice. Interrater and intrarater reliability for each signs and CAT score were evaluated by the intraclass correlation (ICC) and kappa statistic. Results: Interrater reliability of CAT score showed the overall weighted kappa statistic (ICC) for session 1 and session 2 was 0.314 (0.841) and 0.303 (0.846), respectively. Intra-rater reliability of CAT score showed the overall weighted kappa statistic (mean ICC) for session 1 and session 2 was 0.515 (0.907). The total interrater and intrarater reliability for CAT score were fair to good reproducibility. Conclusion: CAT could be considered as a simple and reliable scale for the severity of ETR.