The three-dimensional morphology of human tooth fissures and the quantification of mineral distribution in fissure enamel are pertinent to the development and diagnosis of caries. Synchrotron X-ray microtomography was used to measure linear attenuation coefficients (at 25 keV) at high spatial resolution with a volume-imaging element (cubic voxel) of 4.9x4.9x4.9 microm3 in a block from a human premolar that included part of a stained fissure. From the linear attenuation coefficient, the mineral concentration, expressed as gHAp cm-3 (where HAp is stoichiometric hydroxyapatite), was calculated. The mean mineral concentration in bulk enamel was 2.84 gHAp cm-3. Well-defined regions (1.5-2.6 gHAp cm-3), extending up to approximately 130 microm from the base of some narrower lengths of the fissure and up to approximately 50 microm deep from the fissure surface, were attributed to hypomineralization. Other regions of low mineral concentration, some (1.4-2.3 gHAp cm-3) lying within the expected course of the fissure base and some (2.2-2.7 gHAp cm-3) deep to the pit, were also considered to be of developmental origin. However, a diffuse distribution of low mineral concentrations (2.2-2.7 gHAp cm-3) in the pit walls was attributed primarily to demineralization from caries. The fissure contained heterogeneous material (<or=0.5 gHAp cm-3) exhibiting partial mineralization.