An in vitro study was performed to examine the effects of dental fluorosis on microleakage from Class-V ormocer resin restorations in permanent molar teeth. The Thylstrup-Fejerskov Index (TFI) was used to determine the severity of fluorosis of the teeth as TFI=0, TFI=1-3, and TFI=4-6. Thirty noncarious permanent molar teeth from young or middle-age adult patients were used in the study (ten teeth for each fluorosis level). Sixty Class-V cavities (box-shaped) were prepared on the buccal and palatinal/lingual surfaces of the teeth. The ormocer restorations were polymerized by two different polymerization devices (30 by a halogen light unit and 30 by a LED unit) (n=10 for each TFI level). After a thermocycle procedure (5/55 degrees C, 1000X), all teeth were immersed in 0.5% basic fuchsin at 37 degrees C for 24 hr. The teeth were separated longitudinally in a bucco-lingual direction and were then evaluated for dye penetration by examination with a stereomicroscope (25x). Data were analyzed with Kruskal-Wallis H, Mann-Whitney U, and Wilcoxon Signed Range Tests (p<0.05). Microleakage was significantly greater (p<0.5) from restorations in fluorosed teeth, cervically or occlusally. More leakage was observed at cervical margins than occlusal margins (p<0.05). Less leakage was observed in groups polymerized with the LED unit than with the halogen unit (p<0.05).