Background It has been reported that clinical evaluation consistently underestimates the severity of hidradenitis suppurativa (HS). Objective To determine the usefulness of ultrasound as a diagnostic tool in HS compared with clinical examination and to assess the subsequent modification of disease management. Methods Cross‐sectional multicentre study. Severity classification and therapeutic approach according to clinical vs. ultrasound examination were compared. Results Of 143 HS patients were included. Clinical examination scored 38, 70 and 35 patients as Hurley stage I, II and III, respectively; with ultrasound examination, 21, 80 and 42 patients were staged with Hurley stage I, II and III disease, respectively (P < 0.01). In patients with stage I classification as determined by clinical examination, 44.7% changed to a more severe stage. Clinical examination indicated that 44.1%, 54.5% and 1.4% of patients would maintain, increase or decrease treatment, respectively. For ultrasound examination, these percentages were 31.5%, 67.1% and 1.4% (P < 0.01). Concordance between clinical and ultrasound intra‐rater examination was 22.8% (P < 0.01); intra‐rater and inter‐rater (radiologist) ultrasound agreement was 94.9% and 81.7%, respectively (P < 0.01). Limitations The inability to detect lesions that measure ≤0.1 mm or with only epidermal location. Conclusion Ultrasound can modify the clinical staging and therapeutic management in HS by detecting subclinical disease.