A process for the preparation of aqueous polymer dispersions comprising at least one lipophilic active substance by emulsion polymerization of ethylenically unsaturated monomers in an aqueous medium in the presence of at least one lipophilic active substance, wherein (i) at least one lipophilic active substance which has a water solubility of not more than 5 g/l (determined at 25° C. and 1013 mbar) and a melting point below 130° C. is emulsified in an aqueous solution which comprises at least one dispersion stabilizer with formation of an aqueous dispersion of the active substance having a mean particle size of not more than 1000 nm and (ii) a monomer composition which comprises at least 80% by weight of a neutral, monoethylenically unsaturated monomer emulsifiable in water is subjected to an emulsion polymerization in the aqueous dispersion of the active substance which is obtained according to (i), and use of the dispersions thus obtainable as sizes for paper, board and cardboard, for the treatment and for the stabilization of thermoplastic polymers, for material protection and for crop protection.