Apart from personal fitness, the main reason for failure is the weather. Being so high, it does not take much in the way of bad weather to make conditions on Mont Blanc intolerable, and pushing on in bad conditions greatly reduces safety margins should any problem occur. Having said this, summer alpine conditions are often stable for long periods of time, with afternoon thunderstorms the only concern; these can be spectacular and are best viewed from a Chamonix bar after a successful ascent!

Allocating three days for the main ascent, and staying as flexible as possible regarding time of departure, gives you the best chance of making the top within a six day period. However, there's probably still a 30% chance of bad weather intervening. It is obviously very difficult to be precise about such things, and many operators advertise much more impressive statistics, but a long experience of the mountain (including several periods working for other operators), has shown this to be a reasonable approximation. If you're fit enough and the weather is good, our success rate is 100%!