Severe stratospheric ozone depletion is the result of perturbations of chlorine chemistry owing to the presence of polar stratospheric clouds (PSCs) during periods of Limited exchange of air between the polar vortex and midlatitudes and partial exposure of the vortex to sunlight(1-4). These conditions are consistently encountered over Antarctica during the austral spring. In the Arctic, extensive PSC formation occurs only during the coldest winters, when temperatures fall as low as those regularly found in the Antarctic(1,5,6). Moreover, ozone levels in late winter and early spring are significantly higher than in the corresponding austral season(1,7,8), and usually strongly perturbed by atmospheric dynamics(9-12). For these reasons, chemical ozone loss in the Arctic is difficult to quantify. Here we use the correlation between CH4 and O-3 in the Arctic polar vortex to discriminate between changes in ozone concentration due to chemical and dynamical effects(10). Our results indicate that 120-160 Dobson units (DU) of ozone were chemically destroyed between January and March 1996-a loss greater than observed in Antarctica in 1985, when the ’ozone hole’ was first reported(13,14). This loss outweighs the expected increase in total ozone over the same period through dynamical effects, leading to an observed(6) net decrease of about 50 DU. This ozone loss arises through the simultaneous occurrence of extremely low Arctic stratospheric temperatures(6,15) and large stratospheric chlorine loadings. Comparable depletion is likely to recur because stratospheric cooling(16,17) and elevated chlorine concentrations(5,18) are expected to persist for several decades.