  Let $1\le p<\8$ and $(x_n)_{\nen}$ be a sequence of positive elements in a
non-commutative $L_p$ space and $(E_n)_{\nen}$ be an increasing sequence of
conditional expectations, then the $L_p$ norm of \sum_n E_n(x_n) can be
estimated by c_p times the $L_p$ norm of \sum_n x_n. This inequality is due to
Burkholder, Davis and Gundy in the commutative case. By duality, we obtain a
version of Doob's maximal inequality for $1<p\le \8$.
