To study hygroscopic dimensional changes in new resin-matrix composites during water sorption/desorption cycles.Five materials were examined: a self-adhering flowable composite: Vertise ® Flow (VF), a universal composite: GC Kalore (GCK), two micro-fine hybrid composites: GC Gradia Direct Anterior (GDA) and GC Gradia Direct Posterior (GDP), and a posterior restorative composite: Filtek ® Silorane (FS). Five disk-shaped specimens of each material were prepared (15mm diameter×2mm thickness) according to ISO 4049. The mean diameter of each specimen was measured by a custom-built laser micrometer (to a resolution of 200nm) periodically over 150d water immersion and 40d recondition periods at (37±1)°C. Perspex controls were used. Data analysis was performed by repeated measures ANOVA, one-way ANOVA and Tukey's post hoc test (p<0.05).Differences in hygroscopic expansion were found for all test materials during sorption, ranging from 0.74% (±0.05) for FS to 4.82% (±0.13) for VF. The differences were significant for all materials (p<0.001), except between GCK and GDA. The mathematical relationship between diametral expansion and square root of time was non-linear. VF exhibited significant dehydration shrinkage.The silorane composite FS had the lowest hygroscopic expansion. The extent of compensation of polymerization shrinkage by hygroscopic expansion depends on materials, specimen dimensions and time-scale. So the clinical situation must be taken into consideration in the application of these findings.