Each summer between 1922 and 1938, up to 500 elementary school teachers from across Britain, and some from overseas, joined together in London for a two-week residential vacation course. Organised by Evans' Brothers Publishers and patronised by leading educationists, politicians and policy-makers, the City of London Vacation Course came to be regarded as an important annual educational institution and a cutting-edge exemplar of teacher professional development. In spite of this apparent fame, it appears to have been entirely overlooked in the history of teacher education. This paper seeks to recover the lost story of the City of London Vacation Course and documents its educational and professional focus and its social and cultural function. Locating it within a wider educational, economic and political climate, the paper also examines how the City of London Vacation Course somehow captured and embodied the promise of an emergent new professionalism for elementary teachers during that period. (Contains 78 footnotes.)