Special birational transformations of projective spaces

Extending some results of Crauder and Katz and Ein and Sheperd-Barron on special Cremona transformations, we study birational transformations of P^r onto a prima Fano manifold such that the base locus X in P^r is smooth, irreducible and reduced. The main results are a complete classification when X has either dimension 1 and 2 or codimension 2. Partial results are also obtained when X has dimension 3.