The inherent hydrophilic composition of polyether (PE) impression material and the addition of hydrophilic surfactants to vinyl polysiloxane (VPS) should enhance surface detail reproduction of moist oral tissues. However, impression materials with hydrophilic structures may also be more prone to absorption of moisture and resultant decreased dimensional accuracy. Objectives: The purpose of this investigation was to evaluate and compare the dimensional accuracy and surface detail reproduction of 2 hydrophilic VPS and 2 PE impression materials, when used under dry and moist conditions. Methods: Impressions were made of stainless steel dies as described in ANSI/ADA specification 19, with 2 vertical and 3 horizontal lines inscribed on the die superior surface. Impressions were made under dry and moist conditions (17 impressions/condition for each material). Dimensional accuracy was measured by comparing the average length of the middle horizontal line in each impression to the same line on the metal die, using a measuring microscope with an accuracy of 0.001 mm. Surface detail reproduction was evaluated by using criteria similar to ADA specification 19, continuous replication of at least 2 of the 3 horizontal lines. Results: A 2-way ANOVA indicated that moisture did not cause significant adverse effects on the dimensional accuracy of any material; however, significant differences were found between the materials (p<.05). Only under dry conditions did all materials produce satisfactory detail reproduction 100% of the time. Under moist conditions, only 29% of Aquasil (Dentsply) and Genie Ultra (Sultan) VPS impressions produced satisfactory detail reproduction, while 100% of both Permadyne (3M/ESPE) and Impregum (3M/ESPE) PE impressions met the surface detail criterion. Conclusions: Although moisture may not adversely affect the dimensional accuracy of either PE or hydrophilic VPS material, the evidence suggests that PE material is more likely to produce impressions with superior detail reproduction in the presence of moisture.