With its 10th biannual anniversary conference, Participatory Design (PD) is leaving its teens and must now be considered ready to join the adult world. In this article we encourage the PD community to think big: PD should engage in large-scale information-systems development and opt for a PD approach applied throughout design and organizational implementation. To pursue this aim we extend the iterative PD prototyping approach by (1) emphasizing PD experiments as transcending traditional prototyping by evaluating fully integrated systems exposed to real work practices; (2) incorporating improvisational change management including anticipated, emergent, and opportunity-based change; and (3) extending initial design and development into a sustained and ongoing stepwise implementation that constitutes an overall technology-driven organizational change. The extended approach is exemplified through a large-scale PD experiment in the Danish healthcare sector. We reflect on our experiences from this experiment and discuss four challenges PD must address in dealing with large-scale systems development.