Statement of Problem: If a post does not fit passively, binding with the radicular tooth structure occurs, possibly increasing the risk for root fracture. Therefore, it is necessary to fabricate slightly undersized cast posts to allow for passive fit and cement placement. Shrinkage of the mold cavity may be a desired effect during the casting of posts. Purpose: This study evaluated the influence of: (1) a cellulose ring liner, and (2) a lower casting temperature of the metal ring, on the dimensions of a cast post. Material and Methods: Eighty plastic posts were divided in 4 equal groups (group 1, 815°C, liner; group 2, 815°C, no liner; group 3, 600°C, liner; and group 4, 600°C, no liner), invested in phosphate-bonded investment and cast with a noble alloy. Posts were measured at 2 locations, the tip and base, before and after casting. Results: Investing in a lined metal ring and using a casting ring temperature of 815°C (group 1) resulted in markedly oversized posts (tip: +25.8 μm; base: +20.3 μm). The absence of the cellulose liner in the casting ring (group 2) decreased the expansion of the investment producing slightly oversized castings (tip: +9.75 μm; base: +4.52 μm). Lined rings that were heated to a final temperature of 600°C (group 3) produced posts that were oversized (tip: +16.65 μm; base: +11.05 μm). Final casting ring temperature of 600°C and the absence of a cellulose liner (group 4) resulted in posts slightly undersized (tip: –4.1 μm; base: –2.2 μm). Conclusion: The use of 2 casting ring temperatures (815°C or 600°C) and the influence of a ring liner were investigated. Decreasing the casting ring temperature from 815°C to 600°C and the absence of a ring liner significantly decreased the dimensions of a cast post. (J Prosthet Dent 2000;84:32-7.)