The aim of the present work was to study the influence of surface hydrophilicity of biodegradable polymeric nanoparticles on cellular uptake by Caco-2 cells. Poly(D,L-lactide-co-glycolide acid) particles loaded with a fluorescent dye, 3,3'-dioctadecyloxacarbo-cyanine perchlorate (DiO), were prepared by the emulsion-evaporation process. Three batches of particles with narrow size distribution (100, 300 and 1000 nm) were produced using selective centrifugation. One set of particles was coated by adsorption of chitosan to increase the hydrophilicity of the particles. The interaction of particles with Caco-2 cells was determined by fluorescence spectroscopy and the number of particles associated with one single cell was then calculated. Interaction with cells was clearly dependant on particle size and surface hydrophilicity. Particles in the range of 100 nm presented higher interaction when compared to larger particles. Approximately 6000 uncoated particles and more than 30,000 chitosan-coated particles were quantified per cell. Confocal microscopy confirmed the spectroscopic measurements and revealed the location of the particles in the cell monolayer. Only small particles were observed intracellularly, whereas particles larger than 300 nm were associated with the apical membranes. The location of particles <300 nm appeared to be intracellular and some particles colocalized with the nucleus.