The nomogram devised by Richens and Dunlop for predicting phenytoin dose has been tested in 127 residential epileptic patients, and the data obtained were used to prepare a revised version of the nomogram. In a further 78 patients, this new version was found to be superior to the original. The mean Km value was found to be 23.8 μmoles/liter. Km was independent of age and body surface area, but Dmax correlated positively with the latter two variables.