We describe the implementation of precision laser transmission spectroscopy for sizing and counting nanoparticles in suspension. Our apparatus incorporates a tunable laser and balanced optical system that measures light transmission over a wide (210-2300 nm) wavelength range with high precision and sensitivity. Spectral inversion is employed to determine both the particle size distribution and absolute particle density. In this paper we discuss results for particles with sizes (diameters) in the range from 5 to 3000 nm. For polystyrene particles 404 to 1025 nm in size, uncertainties of ±0.5% in size and ±4% in density were obtained. For polystyrene particles from 46 to 3000 nm in size, the dynamic range of the system spans densities from ~10(3)/ml to ~10(10)/ml (5 × 10(-8) to 0.5 vol. %), implying a sensitivity 5 orders of magnitude higher than dynamic light scattering.