The Union Cycliste Internationale, the governing body for international cycle sport, made cycle helmets mandatory on May 5 2003 although helmet use had been increasing voluntarily since the 1990s. However, there have been more fatalities to cyclists in races since implementation of the helmet rule than in any recent decade (Wiki, 1).
Post compulsion, from May 2003 to 2011 (8.5 years), there have been 9 deaths.
When considering these figures, it should be recognised that medical care has improved greatly over the period and many of the cyclists who died in earlier years may well have survived today. That makes the statistics since 2000 all the more a matter for concern.
Some commentators have suggested that the apparent protection afforded by helmets has encouraged cyclists to take more risks - a form of risk compensation. Also modern helmets can help to decrease aerodynamic drag by approximately 2% over a rider with no helmet, giving a competitive edge in a bicycle race (Wiki, 2).
Compulsory helmet use in cycling sport. Wikipedia, section 2.1.