Gain narrowing in optically pumped thin films, both neat and undiluted, of luminescent conjugated polymers with different molecular structures was demonstrated. These results indicate that the polymers studied have large cross sections for stimulated emission, that population inversion can be achieved at low pump energies, and that the emitted photons travel distances greater than the gain length within the gain medium, The use of simple waveguide structures is sufficient to cause low gain narrowing thresholds in submicrometer-thick films.