Pluricentric randomized study comparing two therapeutic strategies at beginning of Crohn's disease: early immunosuppressants (prescription within the six first months following diagnosis) vs. conventional strategy (immunosuppressants given only in case of steroid failure, in a selected group of patients being at high risk of disabling course. The hypothesis is that immunosuppressants given early may improve the disease course during the 3 following years in this subset of patients.