Hydroxyapatite nanocrystals have been synthesized in the presence of polyacrylic acid in solution. The method leads to apatite crystals with a polyelectrolyte relative content which increases as a function of its concentration in solution and reaches a limit value of about 15% wt. Polyacrylic acid affects the degree of crystallinity, the coherent length of the perfect crystalline domains, as well as the dimensions and the morphology of the crystals. TEM images indicate that on increasing polyelectrolyte content, the dimensions of the crystals are reduced, while their morphology changes from plate-like to acicular, and, at high polyacrylic acid content, the crystals tend to aggregate in clusters. In agreement, the broadening of the X-ray diffraction reflections indicate a reduction of the coherent length along the long dimension (002) and the cross section (310) of the apatite crystals. The reduction is greater along the direction orthogonal to the c-axis, which can be rationalized by a preferential interaction of the carboxylate groups of the polyelectrolyte with the phosphate sites on the crystalline faces parallel to the c-axis.