  The question of the existence of an analogue, in the framework of central
simple algebras with involution, of the notion of Pfister form is raised. In
particular, algebras with orthogonal involution which split as a tensor product
of quaternion algebras with involution are studied. It is proven that, up to
degree 16, over any extension over which the algebra splits, the involution is
adjoint to a Pfister form. Moreover, cohomological invariants of those algebras
with involution are discussed.
