Controversy still exists regarding the paper speed necessary for accurate measurements from records of maneuvers for forced vital capacity. Twenty-four spirometric wave forms of known characteristics were plotted by a computer at 1, 2, and 3 cm/sec and were measured in random order by 12 experienced readers. We found that all readers made a surprisingly large number of major errors. The speed of the paper was found to be an important determinant for accurately measuring the forced expiratory volume in one second and the mean forced expiratory flow during the middle half of the forced vital capacity. A minimum paper speed of at least 3 cm/sec is important if spirograms are to be accurately measured by hand. Human errors in measurement may be minimized by obtaining results from at least three acceptable curves, by making duplicate reading of curves, and by making use of validated computerized measurement systems.