We report measurements of stimulated emission in poly(9,9-dioctylfluorene). We find a gain coefficient of 1.2×104 cm-1 for 100 nm-thick films at an excitation energy density of 9.9 mJ/cm2. Using ultrafast pump-probe measurements, we have studied the effect of cavity optical confinement. A strong shift of the cavity resonance due to nonlinear refraction is observed. © 1999 American Institute of Physics.