  We prove a Bonnet-Myers type theorem for quaternionic contact manifolds of
dimension bigger than 7. If the manifold is complete with respect to the
natural sub-Riemannian distance and satisfies a natural Ricci-type bound
expressed in terms of derivatives up to the third order of the fundamental
tensors, then the manifold is compact and we give a sharp bound on its
sub-Riemannian diameter.
