A defective cemented annulus in a wellbore is repaired by injecting a water-soluble monomeric composition having polymerizable unsaturated groups. This allows repair of leaking cement jobs where the leakage occurs through very small imperfections. In accordance with the invention, a section of the casing above the area where formation fluid pressure may be entering is sealed off with packer devices, perforated or mill cut and a monomer solution is injected into the cemented annulus.