Objective: This study measured and compared the in vitro microleakage behavior of a compomer restorative material, Dyract (LD Caulk); a conventional glass ionomer, Ketac-Fil (ESPE Premier); and a resin-modified glass ionomer, Fuji II LC (GC America). Method and materials: Class V preparations measuring 2 mm 3 3 mm 3 2 mm were cut at the cementoenamel junction on the facial surfaces of 30 freshly extracted bovine incisor teeth. The 30 teeth were randomly divided into 3 groups of 10 each, restored using the material of choice following the manufacturer's instructions, and placed in distilled water at 37°C. After 10 days' storage, the teeth were thermocycled between 4°C and 58°C for 100 cycles using a 1-minute dwell time, followed by a 2-hour immersion in Ca45 isotope. The teeth were sectioned longitudinally, wet-ground until smooth using 600 grit SiC paper, and placed on a radiographic film for 12 hours to produce autoradiographs. Microleakage was evaluated on a 0 (no microleakage) to 3 (gross microleakage) scale with separate scores for the incisal and gingival margins. Results: The results were analyzed using the Mann-Whitney U Test. Results showed no significant differences between groups at the incisal margin (P = 0.10). At the gingival margin, the mean leakage score for Dyract was significantly greater than Fuji II LC and Ketac-Fil (P = 0.08 and P = 0.06, respectively). Conclusion: These results suggest that this new compomer composition may not be as effective as conventional and hybrid ionomers at sealing gingival margins in Class V restorations.