This article explores the challenges faced by union involvement in work-family life balance. It draws on the French case, where the state’s explicit call for firms to address the work-family life balance issue, notably through mandatory collective bargaining, has provided opportunities for union mobilisation. Findings from qualitative research conducted at branch and company levels show that this public strategy has failed to stimulate innovation in related union policy and practices, even within a relatively feminised and partnership-oriented confederation. The reasons for this failure are discussed using a theoretical framework combining opportunity structure, and voice and choice issues. The article concludes that union underinvestment in work-family life balance results from a lack of effectiveness of the opportunity structure, but also, more specifically, from voice and choice factors which lead union representatives to downplay the issues at stake in this new bargaining agenda.