Cutaneous lupus erythematosus (CLE) is a clinically heterogeneous group of rare skin diseases that only rarely have been subjected to controlled clinical trials. This may have been partly due to a lack of suitable validated outcome instruments. Recently, the Food and Drug Administration (FDA) mandated that organ-specific trials for lupus erythematosus need to use a combination of different outcome measures. The patient's condition needs to be assessed in terms of quality of life, the patient's global response, and organ-specific instruments that measure activity of the disease as well as damage due to the disease. For the skin, the only formally validated and published instrument is currently the Cutaneous Lupus Erythematosus Disease Area and Severity Index (CLASI). This paper discusses the background of the development of the CLASI as well as issues related to its use and interpretation in the context of clinical research of CLE.