To determine the effect of pressure variations on the retention of prosthetic crowns, 36 identical, cast metal crowns were fixed to simulated, epocy resin crown preparations with zinc oxyphosphate cement. Eighteen specimens were placed in a hyperbaric chamber and subjected to 15 cycles of simulated immersion at 30 m and decompression, as usually carried out by scuba divers. The untreated specimens served as controls. When the force required to dislodge the crowns was measured, the treated crowns were dislodged at about one third the load that was required to dislodge the control group, indicating that a statistically significant decrease in cement strength follows pressure variations.