In general, the dimensions of the effective focal spot of an x‐ray tube vary with tube current and voltage. This dependence on tube operating conditions has been noted in the literature but has not been investigated in detail. Star phantom measurements of seven focal spots for four x‐ray tubes operated at a variety of tube voltages and currents show that effective focal‐spot size varies asIV − 3 / 2. This dependence may be suppressed for the focal‐spot dimension perpendicular to the tube axis, however, because of auxiliary electrostatic focusing. Experimental data are presented with a theoretical explanation of the relationship between effective focal‐spot size and tube operating conditions.