When the mass arrest of more than 13,000 Jews, the Vel' d'Hiv Roundup, was carried out in Paris on 16 July 1942, Léon's parents sent him and his two brothers into hiding with a neighbour. His parents were arrested; his father was held at Beaune-la-Rolande and Camp of Royallieu transit camps before being deported to Auschwitz concentration camp on 31 July 1942. His mother and two brothers (Henry, 11 and Max, 8) were held at the Pithiviers and Drancy transit camps, and deported to Auschwitz on 19 August 1942.