The sacroiliac (SI) joint is the point of articulation between the sacrum and the innominate bone of the pelvis. The anterior portion is synovial, and the posterosuperior portion is a typical syndesmosis. The SI joint may be affected by inflammatory or degenerative changes which include narrowing, minor sclerosis and erosion. Alterations in the anatomy of this joint may be involved in causing lower back pain. Radiodiagnosis of the width of the articular space is one of the tools utilized to evaluate the normal state of any joint. Scanty data is available on the morphometric characteristics of the SI joint. Therefore, the articular width space of the SI joint was measured to define its normal size in relation to age and sex. The study included 198 x-rays of the abdomen, in anteroposterior projection, of 112 males and 86 females (age range: 17 to 91 yrs). Individual films were divided into four classes according to age (< 50; 50-59; 60-69; > 70). The articular space of the SI joint is made up of a lateral and a medial part: the first representing the anterior articular space; the latter the posterior one. The anterior articular space of both the right and left joint, which was more easily visible and measurable was measured at the medium and inferior levels. Measurements were taken from the radiograms using a semiautomatic computerized system (Videoplan II, Image Analysis system-Kontron). The mean value of the articular space width a the medium and inferior levels, according to age group, was measured in both the males and females. The values obtained were studied using variance analysis. No significant differences were found in the mean values of the articular space width at the SI joint, when males and females were compared in relation to age. Moreover, the articular space width diminished with increasing the age. This fact was not always statistically significant.