Feline dermatology lacks a reliable clinical scoring system. A Canine Atopic Dermatitis Extent and Severity Index (CADESI) is widely used. Cats, however, develop well-defined reaction patterns: symmetrical alopecia (SA), eosinophilic plaque (EP), miliary dermatitis (MD), and head and neck dermatitis (HND). This might preclude comparisons between cats with different lesions of a similar aetiology. A pilot study was therefore undertaken using a modified CADESI termed the Feline Dermatitis Extent and Severity Index (FeDESI). Cats with SA (n = 11), EP (n = 11), MD (n = 5) and HND (n = 3) were recruited. The aetiologies included atopic dermatitis (n = 8), adverse food reactions (n = 2), flea allergic dermatitis (n = 8), psychogenic alopecia (n = 2) and idiopathic (n = 10). Erythema, excoriation and alopecia were graded 0 (normal), 1 (mild), 3 (moderate) or 6 (severe) at 42 body sites to give a final score from 0 to 756. The clinical grading was repeated 2–4 h later for each cat. All cats were assessed by the same investigator and the two scores were blinded. The mean (SD) scores were: SA 50.6 (18.4) and 52.8 (21.8); EP 63.9 (21.5) and 64.4 (19.0); MD 47.2 (17.4) and 43.4 (24.0); HND 57.3 (23.2) and 58.0 (29.5). There were no significant differences between different lesions or between the two scores for each lesion. The proposed FeDESI is therefore an accurate and reliable way for a single investigator to objectively grade the severity of feline skin lesions.