Positive optical gain under pulsed excitation in oxidized porous silicon planar waveguides impregnated with Nile blue (LC 6900) is reported. Amplified spontaneous emission measurements show a dramatic line narrowing when the pump energy is increased, together with a strong superlinear behavior. Variable stripe length measurements were performed to characterize quantitatively the amplification, and an unambiguous transition from losses to gain is observed with a threshold of ~3 mJ/cm2 at 700 nm. A maximum gain coefficient of 8.7 cm−1 (~40 dB/cm) is reported. Shifting excitation spot measurements confirm the reliability of our results. This system is interesting in view of an optically pumped silicon-based pulsed laser.