Most of the methods that are used to produce pharmaceutical suspensions of nanoparticles for drug-targeting yield suspensions having a low content in drug carriers. This can be a dramatic limitation when the volume of suspension that would have to be administered in vivo to reach therapeutic concentrations of the drug is much above the acceptable range. Concentrating the drug-carrier suspension by centrifugation, lyophilization and evaporation is often inapplicable because aggregates are formed. Here we present a simple method that is able to increase the concentration of nanoparticle suspensions without forming aggregates. It consists in a dialysis of the suspensions against a polymer solution. This causes an osmotic stress, which produces a displacement of water from the nanoparticle suspension towards the counter-dialysing solution. Various types of nanoparticle suspensions can be concentrated in near equilibrium conditions, and the result is controlled and reproducible. Concentration factors up to 50 were obtained in a few hours at room temperature. The original characteristics of the nanoparticles were fully preserved in the concentrated dispersion.