A relation is developed between tolerance dose and field size from an analysis of available clinical data on skin-tolerance doses for various treatment field sizes in radiotherapy. It is shown that plots of log-dose against log-field-size for various fractionation schedules give isoeffect lines all having approximately the same slope. This suggests a power law relation between tolerance dose and field size. A generalized form of NSD equation, incorporating the field-size dependence of tolerance dose, is shown to follow naturally when this new relation is combined with Ellis' NSD relation. Certain discrepancies between the sets of data used in the present analysis are pointed out and it is suggested that more data are needed before the generalized NSD relation can be applied under varying conditions of dose, time, fractionation, and field size.