A new miniature Wright peak-flow meter has been designed and produced. The meter is tubular with a spring-loaded piston and a longitudinal slot through which air escapes. Its dynamic characteristics have been carefully designed to make it respond only to peak flow and not to rate of rise. Performance tests on early instruments showed fairly close correlation with the Wright peak-flow meter but with a constant error of + 38 1/min. On later models the correlation was increased to 0.990 and the error reduced to + 3%. The mini-meter correlates as well with the standard instrument as two standard instruments correlate with each other and should prove useful clinically.