Abstract. If (dn) n≥0 is a martingale difference sequence, (εn) n≥0 a sequence of numbers in {1, −1}, and n a positive integer, thenHere αp denotes the best constant. If 1 ≤ p ≤ 2, then αp = 2/Γ(p + 1) as was shown by Burkholder. We show here that αp = p p−1 /2 for the case p > 2, and that p p−1 /2 is also the best constant in the analogous inequality for two martingales M and N indexed by [0, ∞), right continuous with limits from the left, adapted to the same filtration, and such that [M, M ]t −[N, N ]t is nonnegative and nondecreasing in t. In Section 7, we prove a similar inequality for harmonic functions.