We present a tunable microfluidic dye laser fabricated in SU-8. The tunability is enabled by integrating a microfluidic diffusion mixer with an existing microfluidic dye laser design by Helbo et al. By controlling the relative flows in the mixer between a dye solution and a solvent, the concentration of dye in the laser cavity can be adjusted, allowing the wavelength to be tuned. Wavelength tuning controlled by the dye concentration was demonstrated with macroscopic dye lasers already in 1971, but this principle only becomes practically applicable by the use of microfluidic mixing. With presently available dyes, the lasing wavelength can be tuned in an interval between 400 nm and 900 nm, depending on the specific dye. In this first demonstration, the lasing wavelength was tuned between 568 nm and 574 nm, using a solution of 10/sup -2/ mol/L Rhodamine 6G in ethanol.