The history of the French mental-hygiene movement has long been confined to the interwar period. It has focused on the role of Dr Édouard Toulouse, regarded as the founder of the country’s first open psychiatric unit. This article seeks to reposition this narrative in the long term by investigating the experiments conducted on the ground at the beginning of the twentieth century and, more particularly, the origin and operation of a little-known establishment, the psychiatric hospital at Esquermes in Lille, in the Nord department of France. Conceived in 1907, opened in 1912 and closed in 1944, this hospital remained on the margins of the mental-hygiene movement and has been largely forgotten by historians. Nevertheless, it marks an important stage in the French debate on so-called open services and, more generally, the reform of the legal regime governing the treatment of the mentally ill.