Heike Kamerlingh Onnes (1853–1926) was famous for the Cryogenic Laboratory he built up at Leiden University. Hendrik Antoon Lorentz (1853–1928), Professor of theoretical physics at the same university and Onnes' closest colleague, left Leiden in 1912. According to Lorentz's daughter, this move had been initiated by a ‘trick’ Onnes had played on her father a few years earlier. Lorentz had been given two small laboratories for his personal use, but within a short time, according to Lorentz' daughter, Onnes just pinched those rooms and added them to his big laboratory. How did Kamerlingh Onnes and Lorentz get along together, and what really happened in the ‘case of the stolen rooms’?