Background: Epidemiology data regarding hidradenitis suppurativa (HS) are conflicting and prevalence estimates vary 80-fold, from 0·05% in a population-based study to 4%. Objectives: To assess the hypothesis that previous population-based studies underestimated true HS prevalence by missing undiagnosed cases. Methods: We performed a population-based observational and case–control study using the U.K. Clinical Practice Research Datalink (CPRD) linked to hospital episode statistics data. Physician-diagnosed cases in the CPRD were identified from specific Read codes. Algorithms identified unrecognized ‘proxy’ cases, with at least five Read code records for boils in flexural skin sites. Validation of proxy cases was undertaken with general practitioner (GP) questionnaires to confirm criteria-diagnosed cases. A case–control study assessed disease associations. Results: On 30 June 2013, 23 353 physician-diagnosed HS cases were documented in 4 364 308 research-standard records. In total, 68 890 proxy cases were identified, reduced to 10 146 criteria-diagnosed cases after validation, extrapolated from 107 completed questionnaires (61% return rate). Overall point prevalence was 0·77% [95% confidence interval (CI) 0·76–0·78%]. An additional 18 417 cases had a history of one to four flexural skin boils. In physician-diagnosed cases, odds ratios (ORs) for current smoker and obesity (body mass index > 30 kg m-2) were 3·61 (95% CI 3·44–3·79) and 3·29 (95% CI 3·14–3·45). HS was associated with type 2 diabetes, Crohn disease, hyperlipidaemia, acne and depression, and not associated with ulcerative colitis or polycystic ovary syndrome. Conclusions: Contrary to results of previous population-based studies, HS is relatively common, with a U.K. prevalence of 0·77%, one-third being unrecognized, criteria-diagnosed cases using the most stringent disease definition. If individuals with probable cases are included, HS prevalence rises to 1·19%.