The consequences of immobilization error are explored through a model involving the tumor dose-response characteristics. The quantitative effect of such an error depends upon the field size, the shape of the dose-response function, and the position of the nominal dose being delivered on the dose-response function. Squamous-cell carcinoma of the supraglottis and Hodgkin's disease are discussed and values are calculated for the change in tumor control probability which might accompany immobilization error.