Strong autofluorescence from living tissues, and the scattering and absorption of short-wavelength light in living tissues, significantly reduce sensitivity of in vivo fluorescence imaging. These issues can be tackled by using imaging probes that emit in the near-infrared wavelength range. Here we describe self-luminescing near-infrared-emitting nanoparticles employing an energy transfer relay that integrates bioluminescence resonance energy transfer and fluorescence resonance energy transfer, enabling in vivo near-infrared imaging without external light excitation. Nanoparticles were 30-40 nm in diameter, contained no toxic metals, exhibited long circulation time and high serum stability, and produced strong near-infrared emission. Using these nanoparticles, we successfully imaged lymphatic networks and vasculature of xenografted tumours in living mice. The self-luminescing feature provided excellent tumour-to-background ratio (>100) for imaging very small tumours (2-3 mm in diameter). Our results demonstrate that these new nanoparticles are well suited to in vivo imaging applications such as lymph-node mapping and cancer imaging.