Adult atopic dermatitis (AD) is associated with overweight, obesity and cardiovascular diseases (CVD) in Americans, similarly to psoriasis, but no increased risk of CVD has been shown in European patients with AD. This study investigated the prevalence and risk of gallstones in adults with AD and in those with psoriasis as a proxy for obesity using nationwide data for all Danish citizens ≥ 30 years of age. Outcome was a diagnosis of gallstones. Odds ratios (ORs) were calculated by logistic regression (cross-sectional study) and hazard ratios (HRs) were estimated by Cox regression (cohort study). The study comprised 6,742 patients with AD, 53,810 patients with psoriasis, and 3,534,164 general population subjects. The prevalence of gallstones was 3.8%, 3.5% and 5.0% in the general population, AD and psoriasis patients, respectively. Adjusted ORs were 0.81 (0.71–0.92) for AD and 1.18 (1.14–1.23) for psoriasis. During follow-up, adjusted HRs were 0.72 (0.56–0.90) for AD and 1.10 (1.02–1.18) for psoriasis. The findings highlight important differences in obesity and lifestyle factors among patients with AD and those with psoriasis.