Purpose. This study evaluated the cement thickness under crowns cemented with zinc phosphate cement that had been in clinical use for at least 10 years. Material and methods. A total of 97 teeth were extracted because of periodontal disease as part of a new treatment plan. Teeth were sectioned and the cement thickness measured at predetermined points on the occlusal and axial walls with a measuring microscope at ×150 magnification. Results. Mean cement thicknesses on buccal and lingual walls were similar (116 μm and 109 μm, respectively), but much smaller than on occlusal walls (310 μm). Cement thicknesses at occlusal locations were significantly thicker in molars than in premolars. Poor correlations existed between cement thicknesses on buccal and lingual walls (0.26) and between these and the occlusal walls (0.35) implying that incomplete seating is strongly related to tilting or oblique seating. Conclusion. Under clinical conditions where more inherent difficulties exist, special measures must be taken to alleviate the consequences of cement fluid dynamics. (J Prosthet Dent 1998;79:621-5.).