This study evaluated the changes in flexural properties of microfill (Filtek A110 [AO]), minifill (Filtek Z100 [ZO] and Z250 [ZT]), poly-acid modified (F2000 [FT]), and flowable (Filtek Flowable [FF]) composites after aging in water. The flexural strength and modulus of the composites were determined after one week and one month of aging in water at 37 degrees C. Samples were prepared and tested according to ISO specifications. Data was analyzed using ANOVA/Scheffe's test and independent samples t-test at significance level 0.05. Mean flexural strength (n=7) ranged from 66.61 to 147.21 and 68.74 to 142.69 MPa at one week and one month, respectively. Mean flexural modulus (n=7) at one week and one month ranged from 3.45 to 11.30 and 4.76 to 13.02 GPa, respectively. ZO and ZT were significantly stronger than AO, FT and FF and FF was significantly stronger than AO & FT at both time periods. At one week and one month, AO and FF were significantly more flexible than the ZO, ZT and FT. In addition, ZO and FT were significantly stiffer than ZT. With the exception of AO, a significant increase in flexural modulus was observed with all composites. Although flexural strength of FT and FF was significantly increased with aging in water, the flexural strength of ZT was significantly decreased.