An analytic and measurable beam parameter is defined for beams emerging from heterojunction monomode diode lasers with very thin active layers, which provides a joint description of their focusing capabilities and far field divergence behaviour. This parameter can be of use as a figure of merit to compare either the output responses of this type of laser or the different optical systems employed to modify the laser beam. The minimum value of this parameter is determined. Also, it is shown that the beams emitted by these lasers cannot be transformed into circularly symmetric ones by anyABCD optical system. Finally, the possibility of achieving circularly symmetric intensities (at least to second order) and/or beam collimation at some plane is investigated, and its connection with the coupling efficiency between a monomode diode laser and an optical fibre is pointed out.