Adsorption of water was studied gravimetrically at 23 degrees C in an open system at several relative humidities on a variety of apatitic calcium phosphates including enamel, deproteinized enamel, and bone mineral. The amount of adsorbed water increases linearly with the surface areas of the synthetic apatites and does not appear very sensitive to calcium to phosphorus ratio of the apatites. The adsorption results correlate very well up to about two monolayers with a conventionally determined isotherm. Higher uptake of water even by "deproteinized" enamel or bone may be due to the presence of pore structure and incompletely removed organic matter.