Photonic-bandgap materials, with periodicity in one, two or three dimensions, offer control of spontaneous emission and photon localization1, 2, 3. Low-threshold lasing has been demonstrated in two-dimensional photonic-bandgap materials, both with distributed feedback and defect modes4, 5, 6, 7, 8, 9. Liquid crystals with chiral constituents exhibit mesophases with modulated ground states. Helical cholesterics are one-dimensional, whereas blue phases are three-dimensional self-assembled photonic-bandgap structures10. Although mirrorless lasing was predicted11,12 and observed in one-dimensional helical cholesteric materials13, 14 and chiral ferroelectric smectic materials15, it is of great interest to probe light confinement in three dimensions. Here, we report the first observations of lasing in three-dimensional photonic crystals, in the cholesteric blue phase II. Our results show that distributed feedback is realized in three dimensions, resulting in almost diffraction-limited lasing with significantly lower thresholds than in one dimension. In addition to mirrorless lasing, these self-assembled soft photonic-bandgap materials may also be useful for waveguiding, switching and sensing applications.