In 1976, Boston had one of the worst unaccounted-for water problems in the country--about 50%. The Boston Water and Sewer Commission's response included rehabilitating the distribution system, upgrading meters, and an aggressive leak detection and repair program. This paper summarizes the efforts in leak detection and repair. In 1984, 900 miles of pipe were surveyed; 775 leaks losing 10.5 mgd were found. By 1984 many of the mains that leaked most often were replaced. Since then, the commission has instituted a program that includes verifying the accuracy of master meters, measuring consumption over a 24-h period in specific sections of the system, performing sonic surveys in problem areas, and where leaks still appear after repair, street-by-street measurements to pinpoint additional leaks. Leaks continue to be found: 888 in 1988 and 444 in 1989. Major leaks are repaired within 2 days, others are prioritized and repaired within 3 months. In addition to leak detection and repair, the commission regularly tests master meters and has an ongoing main rehabilitation program.