The influence of pressure on the crystallization and annealing of polylaurolactam, nylon-12, has been investigated. The increase of the final melting temperature of this polyamide with pressure amounted to 20°C per kbar as determined by high pressure dilatometry. Crystallization as well as annealing under pressure led to a partial transformation of the pseudo-hexagonal or monoclinic crystal structure to an alpha modification. Samples crystallized at a pressure of 4.9 kbar (1 kbar = 100 MN/m 2) displayed multiple melting behaviour, whereas annealing under pressure gave rise to one melting peak in the d.s.c. thermograms. The heat of fusion could be enhanced from 16 to 32 cal/g and the melting peak temperature could be increased from 179° to 209°C by annealing under 4.9 kbar and 260°C for 336 h. Small-angle X-ray scattering curves reveal that annealing brings about considerable broadening of the distribution of the crystal dimensions. The pressure treated nylon-12, consisting of well developed spherulites, could be fractured very easily along inter-spherulitic and trans-spherulitic planes. The striations in the fracture surfaces as observed in the electron microscope were arranged perpendicular to the radial arms of the spherulites. Annealing at 320°C and 10 kbar for 48 h caused efficient crosslinking of the polylaurolactam.