Objective: Microhardness values of upper and lower surfaces of disk specimens of 3 commercially available resin composites were compared and cured using both the traditional halogen source and a plasma arc lamp. Method and materials: Twenty samples of each of the resin composites were cured in nylon washers. Ten samples of each composite were cured using the appropriate halogen lamp protocol, and 10 samples of each were cured using the plasma lamp protocol. Surface microhardness measurements were carried out using a calibrated Vickers indenter on both top and bottom surfaces after 7 days of storage in air at 20°C. Statistical analysis of the data was carried out using a commercial statistical package. The data for top and bottom surfaces were analyzed separately using a repeated measures analysis of variance (ANOVA). Results: Lamp, composite, and the interaction between lamp and composite were significant in determining the results for both top and bottom surfaces of the disk-shaped specimens. The plasma lamp yielded lower hardness values for all surfaces compared with the halogen source, but the size of the difference depended on the composite. Conclusion: Caution must be exercised in the plasma irradiation of resin composites as a curing method due to the possibility of reduced surface microhardness values that may reflect a reduced percent conversion of monomer to polymer.