Purpose: Previously, we determined a planning target volume (PTV) margin recipe for geometrical errors in radiotherapy equal to M T = 2Σ + 0.7σ, with Σ and σ standard deviations describing systematic and random errors, respectively. In this paper, we investigated margins for organs at risk (OAR), yielding the so-called planning organ at risk volume (PRV). Methods and Materials: For critical organs with a maximum dose (D max ) constraint, we calculated margins such that D max in the PRV is equal to the motion averaged D max in the (moving) clinical target volume (CTV). We studied margins for the spinal cord in 10 head-and-neck cases and 10 lung cases, each with two different clinical plans. For critical organs with a dose–volume constraint, we also investigated whether a margin recipe was feasible. Results: For the 20 spinal cords considered, the average margin recipe found was: M R = 1.6Σ + 0.2σ with variations for systematic and random errors of 1.2Σ to 1.8Σ and −0.2σ to 0.6σ, respectively. The variations were due to differences in shape and position of the dose distributions with respect to the cords. The recipe also depended significantly on the volume definition of D max . For critical organs with a dose–volume constraint, the PRV concept appears even less useful because a margin around, e.g., the rectum changes the volume in such a manner that dose–volume constraints stop making sense. Conclusion: The concept of PRV for planning of radiotherapy is of limited use. Therefore, alternative ways should be developed to include geometric uncertainties of OARs in radiotherapy planning.