Some institutions use people as the material of their action and have the project of transforming individuals. This article aims to show that a commercial weight loss group, insofar as its explicit aim is to make members lose weight, is a genuine ‘people changing institution’, or more specifically a people thinning institution. Based on a 14-month ethnographic study of a commercial weight-loss group, this article analyses how such an organization operates. The article shows that it relies on specific modes of surveillance aimed at controlling the participants’ attitudes and behaviors, and insists on the role played by writing and written materials as key tools in enforcing body discipline. It also argues that the change at issue is class-oriented, and more precisely is one oriented towards the middle and upper regions of the social space. These findings have implications for research on institutions, socialization and individual change.