The classical Wiener index, W(G), is equal to the sum of the distances betw
een all pairs of vertexes of a (molecular) graph, G. We now consider a rela
ted topological index, pi(G), equal to the product of distances between all
pairs of vertexes of G. The basic properties of the pi index are establish
ed and its possible physicochemical applications examined. In the case of a
lkanes, pi and W are highly correlated; a slightly curvilinear correlation
exists between In pi and W.