The importance of mutual repulsion of beam electrons is investigated for conditions relevant to electron beam lithography. A Monte Carlo calculation has been performed which evaluates the spot size for a given beam current and column geometry. The results show that the beam broadening due to mutual repulsion significantly limits the current density obtainable. Experimental results are presented, and the agreement with theory is excellent. For a column 21.5 cm long operating with a 3 mrad aperture semi-angle, the spot size was measured as .14 μm at 100 nA, and .30 μm at 1 μA of beam current. The theory predicts that an optimum spot size exists that is proportional to the 1/4 power of the lens spherical aberration coefficient, and to the 3/4 power of the beam current.