Starting from a hierarchically substructured, heavily dislocated, and highly alloyed martensitic structure, an 18Ni maraging steel was deformed by four passes of equal-channel angular pressing at ambient temperature. X-ray diffraction peak profile analyses according to the modified Williamson-Hall and Warren-Averbach methods were used for determination of apparent grain size, dislocation density, and character of the prevailing dislocations, aided by supplemental transmission electron microscopy. A mean grain size of about 60 nm was determined, corresponding reasonably to the mean dislocation cell size illustrated by means of transmission electron microscopy. Furthermore, a dislocation density of 1.3 x 10(16) m(-2) along with an about 5:1 ratio of screw to edge type dislocations were identified. A dislocation arrangement parameter larger than unity was determined for the present deformed structure, representing a weak dipole character of the dislocation structure and weak screening action of the strain fields of multiple dislocations.