Background: The anti-tumour necrosis factor (TNF)-α adalimumab is the only licenced biologic for moderate-to-severe hidradenitis suppurativa (HS). No predictors of response have been identified so far. Objectives: To identify clinical parameters predicting response to adalimumab and confirm its efficacy/safety. Methods: The data of 389 patients with HS treated with adalimumab in 21 Italian centres were reviewed. Sex, age at onset/diagnosis/baseline, body mass index, smoking, phenotype, previous treatments, concomitant antibiotics and ‘therapeutic delay’, defined as the time from HS onset to adalimumab initiation, were assessed. Response to adalimumab and its impact on quality of life (QoL) were evaluated using the Hidradenitis Suppurativa Clinical Response (HiSCR) and the Dermatology Life Quality Index (DLQI) or the Visual Analogue Scale for pain (VAS pain), respectively. Logistic regression analysis was performed. Results: The therapeutic delay correlated to lack of response to adalimumab at week 16 [odds ratio (OR) 1·92 for therapeutic delay > 10 years; 95% confidence interval (CI) 1·28–2·89; P = 0·0016). HiSCR was achieved in 43·7% and 53·9% patients at week 16 and 52, respectively. Significant reductions in both DLQI and VAS pain were found between week 16 vs. baseline (P <0·0001 for both) and week 52 vs. baseline (P <0·0001 for both). Previous immunosuppressants inversely correlated to HiSCR at week 52 (OR = 1·74, 95% CI 1·04–2·91, P = 0·0342). Conclusions: Inverse correlation between therapeutic delay and clinical response was found, supporting early adalimumab use and providing evidence for a ‘window of opportunity’ in HS treatment. Adalimumab efficacy and safety were confirmed, along with patients’ QoL improvement. Immunosuppressants could negatively influence the response to adalimumab inducing a switch to non-TNF-α-driven pathways. What is already known about this topic?. Adalimumab is an effective and safe biologic licenced for the treatment of moderate-to-severe hidradenitis suppurativa (HS) after failure of conventional treatments. There are no reliable parameters that predict the clinical response to adalimumab in this disease. What does this study add?. The therapeutic delay, defined as the time from HS onset to adalimumab initiation, significantly correlated to lack of clinical response to this drug, particularly at week 16 of treatment. This study suggests that using adalimumab in the early phases of HS should be highly encouraged.