Background: An increased susceptibility to autoimmune disease has been shown in patients with atopic dermatitis (AD), but data remain scarce and inconsistent. Objective: We examined the co-occurrence of selected autoimmune diseases in adult patients with AD. Methods: Nationwide health registers were used. Adult patients with a hospital diagnosis of AD in Denmark between 1997 and 2012 were included as cases (n = 8112) and matched with controls (n = 40,560). The occurrence of autoimmune diseases was compared in the 2 groups. Logistic regression was used to estimate odds ratios. Results: AD was significantly associated with 11 of 22 examined autoimmune diseases. In addition, AD was associated with having multiple autoimmune comorbidities. Patients with a history of smoking had a significantly higher occurrence of autoimmune comorbidities compared to nonsmokers. Limitations: This study was limited to adult patients with AD. No information about AD severity or degree of tobacco consumption was available. Results from a hospital population of AD patients cannot be generalized to the general population. Conclusions: Our results suggest a susceptibility of autoimmune diseases in adult patients with AD, especially in smokers. While we cannot conclude on causality based on these data, an increased awareness of autoimmune comorbidities in patients with AD may be warranted.