Short sisal fibers (3-55 mm) were either mercerized with NaOH or silanized with (EtO)(3)SiCH=CH2 (1% by mass) and used as reinforcement (5-15% by mass) in biopoly-urethane-matrix composites. The tensile strength and rebound resilience of the composites decreased while their hardness increased with increasing content of the fibers. Neither silanization nor mercerization resulted in any improvement of the composite properties (except for their hardness) at the high content of the fibers, in spite of an improved adhesion between the fibers and the matrix.