Objective: To evaluate the effect of manual condensation versus a combination of manual and mechanical, or automatic, condensation on the gingival marginal seal of packable resin composite restorations. Method and Materials: Two packable composites, Pyramid Dentin (Bisco Dental) and Filtek P-60 (3M Espe), were tested. The study consisted of 37 extracted caries-free human molars and premolars. In each tooth, 2 standardized Class 2 cavities were prepared and restored using the incremental technique; the mesial restoration was condensed by hand instrument, and the distal by a combination of the manual and mechanical condenser (KaVo). Restored teeth were subjected to automatic thermal stresses and cycling load stresses. Teeth were immersed in 0.5% basic fuchsin at 37°C for 96 hours. Samples were sectioned mesiodistally in 3 sections. Dye penetration at the gingival margin was examined. Results: Automatic condensation of Filtek P-60 and Pyramid demonstrated better sealing properties, with a median dye penetration of 0 and 1, respectively (on a dye penetration scale of 0 to 3), compared to manual condensation, median of 2 (P = .043) and 3 (P = .079), respectively. Better sealing ability was shown with P-60 than with Pyramid Dentin when manual condensation was used (P = .037). No significant difference was observed between the 2 products when tested with automatic condensation (P = .066). Conclusion: In this in vitro study the combination of manual and automatic condensation resulted in reduced gingival margin microleakage for both packable composites. Keywords: condensation, marginal seal, packable composite, resin composite