Research has shown that external venting improves the marginal fit of cast crowns by decreasing hydrostatic pressure during seating. In turn, improving marginal fit has been shown to increase the strength of castable glass and other porcelain systems. This study evaluated the effect on the compressive strength of crowns with vent holes placed during and after fabrication. Forty-five artificial crowns were made from each material and divided into three equal groups: (1) crowns without vent holes, (2) crowns with a vent hole placed before casting, and (3) crowns with a vent hole placed using a rotary diamond instrument after casting. Standardized crowns were acid etched, silane treated, and filled with epoxy resin to provide a support base for testing. The crowns were then loaded to failure. One-way analysis of variance showed a significant difference among groups. Scheffe's Multiple Comparison Test was used for discrimination. The Dicor unaltered and fabricated vent group and the Hi-Ceram unaltered group had significantly higher resistance to fracture.