Calcium phosphate cement (CPC) hardens in situ to form hydroxyapatite, conforms to complex cavities, and has excellent osteoconductivity.  However, low strength and lack of macroporosity limit its use.  Recent studies used absorbable fibers to provide strength and create macropores.  OBJECTIVES:  To develop strong CPC using synergistic reinforcement; to determine the relationship between fiber strength and composite strength; and to examine the composite biocompatibility.  METHODS:  Chitosan lactate, a biopolymer, was incorporated into CPC liquid at 15% mass fraction.  A suture fiber was randomly mixed into CPC at 45% volume fraction.  Specimens of 3x4x25 mm³ were immersed in a physiological solution for 1 d to 119 d and tested in three-point flexure.  RESULTS:  After 1 d, unreinforced CPC control had a flexural strength (mean ± sd; n = 6) of (2.7 ± 0.8) MPa.  Incorporation of chitosan or fiber into CPC significantly increased the strength to (11.2±1.0) MPa or (17.7±4.4) MPa, respectively (Tukey's at 0.95).  In contrast, incorporating both chitosan and fiber together dramatically increased the strength to (40.5±5.8) MPa.  After 119 d immersion, the fibers in CPC were dissolved and cylindrical macropores were created for cell infiltration and tissue ingrowth.  Tensile strength of single fibers was (461±36) MPa at 1 d and decreased to 0 MPa at 35 d.  A predictive equation was established between fiber strength, S_F, and composite strength, S_C:  S_C = 14.1 + 0.047 S_F, with R = 0.92.  CPC with fibers or chitosan supported osteoblast cell attachment, proliferation and viability, similar to CPC control.  CONCLUSION: Combining two different reinforcing agents (chitosan and fibers) together in CPC resulted in a dramatic synergistic effect of strengthening, compared to the traditional use of a single reinforcing agent.  The strong and biocompatible CPC may be useful in stress-bearing dental and craniofacial repairs.  Supported by NIDCR DE14190, NIST, and the ADAF.