Background: Initiation of adjuvant chemotherapy within 6–12 weeks after mastectomy is recommended by guidelines. The aim of this population-based study was to investigate whether immediate breast reconstruction (IBR) after mastectomy reduces the likelihood of timely initiation of adjuvant chemotherapy. Methods: All patients with breast cancer who had undergone mastectomy and adjuvant chemotherapy between 2012 and 2016 in the Netherlands were identified. Time from surgery to adjuvant chemotherapy was categorized as within 6 weeks or after more than 6 weeks, within 9 weeks or after more than 9 weeks, and within 12 weeks or after more than 12 weeks. The impact of IBR on the initiation of adjuvant chemotherapy for these three scenarios was estimated using propensity score matching to adjust for treatment by indication bias. Results: A total of 6300 patients had undergone primary mastectomy and adjuvant chemotherapy, of whom 1700 (27·0 per cent) had received IBR. Multivariable analysis revealed that IBR reduced the likelihood of receiving adjuvant chemotherapy within 6 weeks (odds ratio (OR) 0·76, 95 per cent c.i. 0·66 to 0·87) and 9 weeks (0·69, 0·54 to 0·87), but not within 12 weeks (OR 0·75, 0·48 to 1·17). Following propensity score matching, IBR only reduced the likelihood of receiving adjuvant chemotherapy within 6 weeks (OR 0·95, 0·90 to 0·99), but not within 9 weeks (OR 0·97, 0·95 to 1·00) or 12 weeks (OR 1·00, 0·99 to 1·01). Conclusion: Postmastectomy IBR marginally reduced the likelihood of receiving adjuvant chemotherapy within 6 weeks, but not within 9 or 12 weeks. Thus, IBR is not contraindicated in patients who need adjuvant chemotherapy after mastectomy.@en