Our ground-based millimeter-wave measurements of chlorine monoxide (ClO) in the ozone layer above Hawaii since 1992, together with a few measurements from the same site using an earlier version of the same instrument during the 1980's, provide a unique data set for the determination of the long-term trend of active chlorine in the middle stratosphere. The trend shows a rapid rise of 58% from 1982 to a broad maximum in 1994--1997 and a substantial decline of 1.5%/year from the maximum through late 2004. About 1/3 of the decline may be due to changes in methane. Both the rise and the decline are consistent with that expected from the rise and decline of total chlorine in the troposphere, which reached a maximum in 1992--1993, then declined sharply after implementation of international agreements to phase out chlorofluorocarbon (CFC) production starting with the Montreal Protocol.