In situ measurements of ClO in the winter lower stratosphere are presented for six flights of the NASA ER‐2 aircraft from 38°N to 61°N. Enhanced abundances, increasing in severity with date, were observed below 20 km, where HCl and ClONO2dominate the inorganic chlorine budget. The greatest mixing ratios, over 150 pptv, were encountered on February 20 and 21, 1989, as the vortex experienced a major warming. Although the timing of these ClO enhancements and the evidence that vortex air can reach midlatitudes suggest that heterogeneous conversion of chlorine compounds within the vortex influences chemistry at midlatitudes, the enhancements observed early in the winter could have been caused by unknown chemistry occurring outside the vortex. In either case, photochemical loss of ozone due to catalytic reactions involving ClO at these mixing ratios may be responsible in part for the ozone decreases observed at high latitudes in the northern hemisph