Summary Background  Reliable and validated measures of skin disease severity are needed for cutaneous dermatomyositis (DM). Cutaneous Dermatomyositis Disease Area and Severity Index (CDASI), Dermatomyositis Skin Severity Index (DSSI) and Cutaneous Assessment Tool (CAT) skin indices have been developed as outcome instruments. Objectives  We sought to demonstrate reliability and validity of the CDASI, and to compare the CDASI with other potential tools for use in measuring disease severity in cutaneous dermatomyositis. Patients and methods  CDASI has four activity and two damage measures, with scores from 0 to 148. DSSI assesses activity based on body surface area and severity on a scale of 0–72. CAT uses 21 activity and damage items, for a range of 0–175 for activity and 0–33 for damage. Ten dermatologists used the instruments to score the same 12–16 patients in one session. Global validation measures were administered to physicians and patients. Results  Global validation measures correlated with the three outcome instruments (P < 0·0001). CAT displayed lower inter‐ and intrarater reliability relative to the CDASI. All scales correlate better with physician than patient global skin measures. Conclusions  It appears that the CDASI may be a useful outcome measure for studies of cutaneous DM. Further testing to compare responsiveness of all three measures is necessary.