Hušek defines a space X to have a small diagonal if each uncountable subset of X² disjoint from the diagonal has an uncountable subset whose closure is disjoint from the diagonal. Hušek proved that a compact space of weight ω₁ which has a small diagonal will be metrizable, but it remains an open problem to determine if the weight restriction is necessary. It has been shown to be consistent that each compact space with a small diagonal is metrizable; in particular, Juhász and Szentmiklóssy proved that this holds in models of CH. In the present paper we prove that this also follows from the Proper Forcing Axiom (PFA). We furthermore present two (consistent) examples of countably compact non-metrizable spaces with small diagonal, one of which maps perfectly onto ω₁.