OBJECTIVE This investigation evaluated and compared the dimensional accuracy and surface detail reproduction of two hydrophilic polyvinylsiloxane (PVS) and two polyether (PE) impression materials when used under dry and moist conditions. METHODS Impressions were made of stainless steel dies as described in ANSI/ADA specification no. 19, with two vertical and three horizontal lines inscribed on the die superior surface. Impressions were made under dry and moist conditions (17 impressions per condition for each material). Dimensional accuracy was measured by comparing the average length of the middle horizontal line in each impression with 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 no. 19: continuous replication of at least two of the three horizontal lines. RESULTS The mean percent dimensional change and SD values ranged from -0.135% (0.035) to 0.053% (0.031). A two-way analysis of variance (ANOVA) indicated that moisture did not cause significant adverse effects on the dimensional accuracy of any material (p > 0.05); however, significant differences were found between the materials (p < .05). The surface detail evaluation indicated that moisture had a significant effect on detail reproduction of PVS materials (Pearson's Chi square, p < 0.05). Under dry conditions, all materials produced satisfactory detail reproduction 100% of the time; however, under moist conditions, only 29% of Aquasil and Genie Ultra PVS impressions produced satisfactory detail reproduction, while 100% of Permadyne Garant and Impregum Penta Soft PE impressions still met the surface detail criteria. CONCLUSIONS Although moisture may not adversely affect the dimensional accuracy of either PE or hydrophilic PVS material, the evidence suggests that PE material is more likely to produce impressions with superior detail reproduction in the presence of moisture.