Atopic dermatitis (AD) is a chronic, recurrent, inflammatory skin disease which predominantly affects children. However, AD may persist until adulthood (persistent AD), or directly start in adults (adult-onset AD). AD often shows a non-flexural rash distribution, and atypical morphologic variants in adults and specific diagnostic criteria are lacking. Moreover, adult AD prevalence as well as detailed data which can characterize persistent vs adult-onset subtype are scant. The aim of this study was to investigate on the main features of adult AD particularly highlighting differences between persistent vs adult-onset form. An Italian multicentre observational study was conducted between April 2015–July 2016 through a study-specific digital database. 253 adult AD patients were enrolled. Familiar history of AD was negative in 81.0%. Erythemato-desquamative pattern was the most frequent clinical presentation (74.3%). Flexural surface of upper limbs was most commonly involved (47.8%), followed by eyelid/periocular area (37.9%), hands (37.2%), and neck (32%). Hypertension (7.1%) and thyroiditis (4.3%) were the most frequent comorbidities. A subgroup analysis between persistent (59.7%) vs adult-onset AD patients (40.3%) showed significant results only regarding AD severity (severe disease was more common in persistent group, p < 0.05), itch intensity (higher in adult-onset disease), and comorbidities (hypertension was more frequent in adult-onset group, p < 0.01). Adult AD showed uncommon features such as significant association with negative AD family history and lacking of association with systemic comorbidities respect to general population. No significant differences among persistent vs adult-onset subgroup were registered except for hypertension, itch intensity, and disease severity.