The aim of this study was to evaluate the immediate changes in the pulp-dentin complex that result from crown preparation, and their correlation with the thickness of remaining dentin and the preparation technique (with or without water spray cooling). Thirty upper intact premolars scheduled for extraction for orthodontic reasons were high speed prepared, extracted immediately after preparation and divided in 5 groups. The analysis of the pulp morphology demonstrated that there are several differences according with the preparation technique. The most severe changes appear after the profound preparation without water-cooling, the odontoblastic layer being extremely affected. Also, vascular reactions and inflammatory infiltrate (in the absence of bacteria) were present. Our study revealed that the histologic changes in the pulp and dentin following complete crown preparation occur anyway and they are considered difficult to avoid, even if an adequate technique of preparation is used.