Crustal accretion at mid-ocean ridges is generally modelled as a symmetric process. Regional analyses, however, often show either small-scale asymmetries, which vary rapidly between individual spreading corridors, or large-scale asymmetries represented by consistent excess accretion on one of the two separating plates over geological time spans(1-6). In neither case is the origin of the asymmetry well understood. Here we present a comprehensive analysis of the asymmetry of crustal accretion over the past 83 Myr based on a set of self-consistent digital isochrons(7) and models of absolute plate motion(8,9). We find that deficits in crustal accretion occur mainly on ridge hanks overlying one or several hotspots. We therefore propose that asymmetric accretion is caused by ridge propagation towards mantle plumes or minor ridge jumps sustained by asthenospheric flow(10,11) between ridges and plumes. Quantifying the asymmetry of crustal accretion provides a complementary approach to that based on geochemical(12) and other geophysical data(13,14) in helping to unravel how mantle plumes and mid-ocean ridges are linked through mantle convection processes.