We developed and validated a measurement instrument (CLASI—Cutaneous Lupus Erythematosus Disease Area and Severity Index) for lupus erythematosus that could be used in clinical trials. The instrument has separate scores for damage and activity. A group of seven American Dermato-Rheumatologists and the “American College of Rheumatology Response Criteria Committee on SLE (systemic lupus erythematosus)” assessed content validity. After a preliminary session, we conducted standardized interviews with the raters and made slight changes to the instrument. The final instrument was evaluated by five dermatologists and six residents who scored nine patients to estimate inter- and intra-rater reliability in two sessions. Consultation with experts has established content validity of the instrument. Reliability studies demonstrated an intra-class correlation coefficient (ICC) for inter-rater reliability of 0.86 for the activity score (95% confidence interval (CI)=0.73–0.99) and of 0.92 for the damage score (95% CI=0.85–1.00). The Spearman's ρ (Sp) for intra-rater reliability for the activity score was 0.96 (95% CI=0.89 to 1.00) and for the damage score Sp was 0.99 (95% CI=0.97–1.00). Clinical responsiveness needs to be evaluated in a prospective clinical trial, which is ongoing.