We present measurements of the differential gain of multiquantum well (MQW) broad area lasers emitting at 1.55<FONT FACE="Symbol">m</font>m wavelength, taking into account the nonuniform stimulated emission caused by filamentation within the optical cavity. The differential gain was determined from measurements of the threshold current density, considering the losses due to the inhomogenous carrier density distribution, as an apparent leakage current effect in the output power-current characteristics. The lasers were grown by low pressure MOCVD and incorporate an InGaAs/InGaAsP/InP separate confinement MQW active region. The results are compared with previous data reported on similar MQW heterostructures.