Waveguiding properties of InGaN/GaN/AlGaN double heterostructures were modeled. A comparison between double heterostructures (DHs) and separate confinement heterostructures (SCHs) has been made. This is an attempt to consider influence of material absorption in passive layers of nitride laser structures on a threshold current. This absorption significantly increases the threshold current, and is especially important in the case of quantum well SCHs. The threshold current density in nitride-based lasers has been estimated to be about 10 kA/cm2. Due to high absorption of light in passive layers, no decrease in the threshold current of single quantum well SCH lasers in comparison with DH lasers is expected. The threshold current density can be decreased in multiquantum well lasers. In these lasers, the threshold current density is expected to be about several kA/cm2. The optimal thickness of active and adjacent layers as well as alloy compositions in cladding layers have been estimated, and as our simulation has shown, no more than 8&percent;–10&percent; of AlN in claddings are needed. ©1997 American Institute of Physics.