We report a detailed investigation of the optical gain properties of a series of semiconducting polyfluorenes with photoluminescence emission spectra that span the entire visible spectrum. Stimulated emission was demonstrated at low pump pulse energies (≥0.1 μJ for 10 ns, 10 Hz pulses) in the blue, green and red parts of the spectrum via amplified spontaneous emission (ASE) measurements on asymmetric slab waveguides. These structures exhibit large net gains g≤74 cm-1, and corresponding gain cross-sections σ≥7×10-16 cm2, together with very low loss coefficients, α≥3.2 cm-1. The spectral location of the maximum waveguide amplification can be widely tuned (Δλ≥30 nm) by controlling the allowed propagating modes or by blending the active emitter with a second larger optical gap polyfluorene. Our results confirm that fluorene-based polymers are attractive gain media for use in highly tuneable solid-state polymer lasers and amplifiers. © 2003 Elsevier B.V. All rights reserved.