The contribution of flanges to the shear strength of reinforced concrete beams has up to now either been neglected or evaluated by very simple empirical formulas. However, the contribution may sometimes be large, up to 20-30%. In this paper the flange contribution for shear reinforced I-beams has been calculated using a simple upper bound approach according to the theory of plasticity. The upper bound mechanism consists of a frame action, where the frame is supposed to be composed by the flanges and parts of the web. The results of the calculation have been compared with tests covering a wide range of geometrical parameters. The comparisons show that the method suggested does indeed lead to very accurate results.