The co-existence of hidradenitis suppurativa (HS) and atopic dermatitis (AD) had been reported but was not investigated in controlled studies.To evaluate the bidirectional association between HS and AD.A population-based retrospective cohort study was conducted to compare incidence rate of AD among HS patients (n=6,779) and age-, gender- and ethnicity-matched control subjects (n=33,260). Adjusted hazard ratios (HRs) and adjusted odds ratios (ORs) were estimated.The incidence of AD was 2.51 (95% CI, 2.07-3.02) and 1.24 (95%CI, 1.10-1.40) per 1,000 person-years among patients with HS and controls, respectively. Patients with HS were twice as likely to develop AD as controls (HR, 2.06; 95%CI, 1.64-2.58). Furthermore, the prevalence of preexisting AD was higher in patients with HS than in control subjects (2.5% vs. 1.8%, respectively; P<0.001). A history of AD was associated with a 40% increase in the odds of HS (OR, 1.41; 95%CI, 1.19-1.67). Relative to patients with isolated HS, those with a dual diagnosis of HS and AD were younger, had a female predominance, lower prevalence of smoking, and lower body mass index (BMI).Retrospective data collection CONCLUSIONS: A bidirectional association between HS and AD was observed. Dermatologists should be aware of this association.