The purpose of this study was to analyse the flexural strength (FS) and flexural modulus (FM) of bulk-fill composite materials, with an emphasis on clinically relevant polymerisation conditions. Specimens 4-mm-thick were compared with 2-mm-thick specimens prepared according to the ISO standard. Said properties were also tested under the accelerated ageing conditions. Five bulk-fill composite materials were tested, two of which are specifically designed for rapid polymerisation with high light intensity. Cuboid-shaped specimens were made, measuring: 16 x 2 x 4 mm and 16 x 2 x 2 mm. Three polymerisation protocols were used: (I) ISO: 2-mm-thick specimens polymerised 6 x 20s (1,000 mW/cm2, double-sided); (II) 10s: 4-mm-thick specimens polymerised 3 x 10s (1,073 mW/cm2, one-sided); (III) 3s: 4-mm-thick specimens polymerised 3 x 3s (2,589 mW/cm2, one-sided). Three measurement times were defined, that is, the mechanical properties were tested after: one day; thermocycling (10,000 cycles); and thermocycling (10,000 cycles) followed by 7 days in absolute alcohol. The FS was measured by a three-point bending test (Inspekt Duo 5kN-M, Hegewald&Peschke, Nossen, Germany), while the FM was calculated. The FM was significantly higher in specimens polymerised according to the ISO protocol than in 4-mm-thick specimens. The differences in flexural strength values for all polymerisation protocols were equalised after thermocycling and immersion in absolute alcohol. All tested materials met the ISO 4049 flexural strength requirement (80 MPa) for all polymerisation methods and ageing conditions. Rapid polymerisation achieved almost optimal values of the FS of tested materials, similar to those prepared under the ISO protocol.