Statement of problem: The primary factor that determines the prognosis of endodontically treated and restored teeth is preservation of sound dentin. The potential of these teeth to fracture is directly related to the amount of dentin removed. Purpose: The purpose of this study was to measure residual dentin thickness of bifurcated maxillary first premolars, in vitro, after root canal and conservative post space preparation. Material and methods: Human premolars (n=13) with bifurcation at the junction of cervical and middle root thirds were embedded in a muffle device and horizontally sectioned 2, 4, and 6 mm apically to the cement-enamel junction. Residual dentin thickness was measured at the buccal, lingual, mesial, and distal aspects, apically to the bifurcation, and at the buccal or lingual, mesial, and distal aspects, coronally to the bifurcation. This procedure was repeated at baseline, after endodontic preparation (K-40 file), and after preparing the post space (ParaPost drills Nos. 3 and 4). Residual dentin thickness was analyzed using 3- or 4-way repeated measures ANOVA (α=.05). Results: Endodontic therapy and post preparation reduced more dentin in the bifurcation aspects of both roots compared to outer aspects (P=.001). Post space preparation jeopardized the roots due to a residual dentin thickness of less than the recommended minimum 1-mm thickness in 61% (lingual) and 77% (buccal) of roots. Conclusions: Lack of residual dentin thickness after post preparation implies that the use of posts in maxillary first premolars should be limited. When mandatory, it is recommended that the lingual root be used and the buccal root be avoided. (J Prosthet Dent 2008;99:267-273).