Turbidity measurements were used to characterize the effect of shear flow on the crystallization kinetics of several isotactic poly (1-butene) samples of different molecular weight (MW). Polymer melts were rapidly cooled below the nominal crystallization temperature, and subjected to a shear flow of varying shear rate but constant total deformation. While the quiescent crystallization was found to be essentially MW-independent, a strong effect of MW on the flow-induced crystallization kinetics was observed. It was shown that such an effect could be cast in terms of a characteristic Weissenberg number, which measures the ability of flow to orient the polymer chains. The proposed scaling relation was found to predict correctly the dependence of flow-induced crystallization upon molecular weight, at least when samples of similar molecular weight distribution were considered. The molecular weight scaling was also found to explain qualitatively the observed transition from a low-shear rate isotropic morphology to a high-shear rate rodlike crystalline structure.