Background: Acellular dermal matrix (ADM) in implant-based breast reconstructions (IBBRs) aims to improve cosmetic outcomes. Six-month data are presented from a randomized trial evaluating whether IBBR with ADM provides higher health-related quality of life (HRQoL) and patient-reported cosmetic outcomes compared with conventional IBBR without ADM. Methods: In this multicentre open-label RCT, women with breast cancer planned for mastectomy with immediate IBBR in four centres in Sweden and one in the UK were allocated randomly (1 : 1) to IBBR with or without ADM. HRQoL, a secondary endpoint, was measured as patient-reported outcome measures (PROMs) using three validated instruments (EORTC-QLQC30, QLQ-BR23, QLQ-BRR26) at baseline and 6 months. Results: Between 24 April 2014 and 10 May 2017, 135 women were enrolled, of whom 64 with and 65 without ADM were included in the final analysis. At 6 months after surgery, patient-reported HRQoL, measured with generic QLQ-C30 or breast cancer-specific QLQ-BR23, was similar between the groups. For patient-reported cosmetic outcomes, two subscale items, cosmetic outcome (8⋅66, 95 per cent c.i. 0⋅46 to 16⋅86; P = 0⋅041) and problems finding a well-fitting bra (−13⋅21, −25⋅54 to −0⋅89; P = 0⋅038), yielded higher scores in favour of ADM, corresponding to a small to moderate clinical difference. None of the other 27 domains measured showed any significant differences between the groups. Conclusion: IBBR with ADM was not superior in terms of higher levels of HRQoL compared with IBBR without ADM. Although two subscale items of patient-reported cosmetic outcomes favoured ADM, the majority of cosmetic items showed no significant difference between treatments at 6 months. Registration number: NCT02061527 (www.clinicaltrials.gov).