The previously reported prevalence of adult-onset atopic dermatitis (AD) varied from 13% to 47%. There were a few reports of clinical features of adult-onset AD. The purpose of this article was to study the clinical features of Thai patients with adult-onset AD. We recruited prospective patients from the outpatient Department of Dermatology, Siriraj hospital, Mahidol University in Bangkok, Thailand, from June, 2006 to May, 2007. The diagnosis of AD was made according to the criteria of Hanifin and Rajka and the severity of AD in each patient was assessed using the Rajka and Langeland score. Fifty nine patients were enrolled. The majority of the patients developed their dermatitis during their third decade. Typical lichenified/exudative lesions were found in all cases. Non-typical morphologic variants were found in 76.3%. The most common were nummular lesions. The main sites of involvement were the flexural area. The common sites of non-flexural involvement were the trunk, extensors and hands. The severity of AD was moderate in 64.4% of cases. A personal history of atopy was found in 84.7% of cases. Skin prick testing showed positive results (mostly to multiple allergens) in 25 of 29 patients (86.2%). Elevated serum total immunoglobulin E was detected in 6 of 10 patients (60%). It is concluded that adult-onset AD is not a rare but under-recognized eczematous condition.