This work was aimed at the study of the viscoelastic properties of two current light-cured dental resin composites, Rok (hybrid) and Ice (nanohybrid), by dynamic mechanical analysis, under the influence of variable temperatures and frequencies 1 h after curing or after storage at 37 degrees C either in air or distilled water for 1, 7 or 30 days. They both contain about the same amount of filler, which is strontium aluminosilicate particles, but they have different size distribution, 40 nm-2.5 microm for Rok and 10 nm-1 microm for Ice. The resin matrix of Rok consists of UDMA, while that of Ice consists of UDMA, Bis-EMA and TEGDMA. During dynamic testing elastic modulus (E'), viscous modulus (E'') and loss tangent (tan delta) were determined over a temperature range from 25 to 185 degrees C at a frequency of 0.1, 1.0 or 10.0 Hz. The elastic modulus (8.40 +/- 0.61 GPa) and tan delta (0.114) of Rok at 37 degrees C and 1 Hz was slightly higher than that of Ice (E' = 7.58 +/- 0.50 GPa and tan delta = 0.110) which, however, were not statistically different (P < or = 0.05). Both the light-cured composites showed two T(g) values, at 58 and 121 degrees C for Rok and at 63 and 117.0 degrees C for Ice. The first T(g) has an apparent value, since inside the glass transition region an additional curing reaction occurred of the unreacted C=C methacrylate groups; the second T(g) is due to the new material formed during the DMA scan. During the storage of composites in air or water also an additional curing reaction took place. The nanohybrid Ice showed a slight lower elastic modulus and tan delta than the hybrid Rok at 25-50 degrees C, which, however, were not statistically different (P < or = 0.05).