This paper considers the changing concepts in management, and the results of surgery in 1000 isolated procedures on the mitral valve, during the past 25 years.The first 251 patients were managed by closed methods with an operative mortality of 4%, but with a high reoperation rate, at least 44% ultimately having valve replacement. Open methods were first used in 1960, 17 patients having conservative valvotomy without mortality, but with indifferent results. Valve replacement began in March, 1963, and since that time 615 mitral valve replacements have been carried out, in 433 of which it was an isolated procedure. The first 290 patients had replacement with a Starr Edwards ball valve prosthesis. The actuarial curve shows a survival rate of 70% at 5 years, 61% at 8 years and probably 50% at 15 years. The remainder, except for two, had replacement with disc valves, the first 65 with Bjork Shiley, and the last 76 with Lillehei Kaster prostheses. Four patients with Bjork Shiley prostheses developed thrombosis on the prosthesis, and this was a factor influencing the change to a LilleheiKaster prosthesis. The actuarial curve shows a survival rate of 86% at 4 1/2 years. Since March 1963, 171 patients have had conservative procedures with preservation of the valve; in 57 it was a blind procedure and in 114 it was an open procedure. There were no early deaths in these 171 patients, but there have been four late deaths. Twenty later came to valve replacement, 12 (20%) of them being in the closed gr'oup and 8 (7%) in the open group. One hundred and twenty‐eight (74%) are considered to be Grade I or II, most of these being in the open grou