A method for the calculation of optical modes in disk-like resonant cavities is presented. The eigenvalue equation for resonant wavelengths and threshold optical gain as well as the optical field distribution is established. The characteristics of the optical modes in microdisk lasers are studied using this method. Numerical calculations are carried out for some long wavelength microdisk lasers. The theoretical analyses and numerical results show that the optical modes in a microdisk laser with a very small thickness can be approximately classified into transverse electric and transverse magnetic modes, but the lasing modes and other lower threshold modes are transverse electric polarized, and a spontaneous emission coefficient between 0.2 and 0.3 can be achieved in long wavelength semiconductor microdisk lasers. The possibility of a microdisk laser to lase in a surface emitting mode is also demonstrated. ©1997 American Institute of Physics.