A process for the production of natural fibre-reinforced polyisocyanate-polyaddition products (I) by reacting (A) polyisocyanates with (B) isocyanate-reactive compounds and optionally (C) chain extenders and/or crosslinkers, catalysts, blowing agents and other additives etc. in the presence of natural fibres, in which at least 20 wt% of component (B) consists of ring-opened epoxidised fatty acid compounds and/or adducts of fatty acid compounds and alkylene oxides. Also claimed are (i) fibre-reinforced products (I) obtained by this process, (ii) building, packaging and furnishing materials and vehicle construction materials containing (I), and especially (iii) interior linings for vehicles containing (I). Preferably the modified fatty acid compounds in (B) are based on materials of vegetable origin, especially oleic and/or linoleic acid; preferably, at least 20 wt.% of (B) consists of hydroxylated castor oil. Preferred natural fibres are mixtures of sisal with jute, hemp and/or flax. Preferred (I) is in the form of moulded sheet with a thickness of 1-10 mm, possibly laminated with outer layers.