  Let P be a finite metacyclic 2-group and F a fusion system on P. We prove
that F is nilpotent unless P has maximal class or P is homocyclic, i.e. P is a
direct product of two isomorphic cyclic groups. As a consequence we obtain the
numerical invariants for 2-blocks with metacyclic defect groups. This paper is
a part of the author's PhD thesis.
