The aim of this in vitro study was to determine the relative contributions to retention of the different components of the post-retained crown and the mode of failure when the crown was loaded at 50 degrees to the vertical (i.e., in an apical direction). Failure was initiated in all the specimens as a rotation of the crown about a point on the labial surface of the root at the crown-root interface with pull-out of the post. A semi-empirical model was developed to explain this mode of failure. From experimental results, it was noted that the retention provided by the post was the most significant factor in the equation. However, large variation in the maximum forces recorded from pull-out tests on 40 posts suggested that the glass-ionomer cement was acting in the manner of a brittle composite material whose failure may be attributed to defects in the structure causing stress concentrations. The probability of survival at each ultimate strength recorded experimentally was plotted for comparison with the curve generated by the derived Weibull function. Close agreement was noted. Thus, for any given force applied compressively at 50 degrees to the vertical to a post-retained crown of known dimensions, it is possible, by use of the described equation, to calculate the force, P, applied to the post. This value of P may then be inserted into the Weibull equation to give the probability of the crown failing under that applied force.