At the 2004 Summer Olympics in Athens, Lagoun qualified as an individual swimmer for the men's 100 m butterfly. He cleared a FINA B-standard entry time of 53.66 from the European Championships in Madrid, Spain. He challenged seven other swimmers on the fifth heat, including top medal favorite Milorad Čavić of Serbia and Montenegro. He raced to fifth place by 0.24 of a second behind France's Frédérick Bousquet in 53.87. Lagoun failed to advance into the semifinals, as he placed twenty-sixth overall in the preliminaries.