Abstract: Introduction: The association of Behcet's disease (BD) and ankylosing spondylitis (AS) is still a controversial subject. As the presence of sacroiliac joint (SIJ) involvement is an essential criterion in the diagnosis of AS, we decided to determine the prevalence of SIJ involvement in BD and compare it with control group. Materials & Methods: We selected randomly 199 BD patients and 168 non-BD cases (CG), in a 12 month period (between April 1998 to May 1999). All were over 20 year of age. Standard anteroposterior radiographs of the SIJ were obtained and interoreted by two Rheumatologists and a radiologist blinded to the diagnosis. The following 5-points scale was employed: Normal (0), pseudo-widening (I), sclerosis (2), erosion (3), and bony fusion (4). To eliminate any doubt about sacroiliitis only grade 3 and 4 were accepted as SIJ involvement. Both groups were evaluated according to their age (before 30/over 30), and sex separately. The mean result in each group (BD/CG) was compared by the chi square test. Results: Both groups were sex and age matched: There was 98 females in BD vs. 91 in CG (p=0.35). The mean age in BD was 34.9±8.3 vs. 34.9±10 in CG (p=1). SIJ was involved in 9 patients in BD (4.6%, CI:2.9) vs. 7 patients in CG (4.2%, CI:3). The difference was not statistically significant (p=0.93). Comparing the results seperately in males and females, there was not statistically significant difference (p=0.68 in males, p=0.64 in females). Another sub-division according to the age (under and over 30) again showed no significant difference between the 2 groups. There was 64 patients under 30 in BD with 3 SIJ involvement vs. 61 patients in CG with 2 SIJ involvement (p=0.96). In patients over 30, 6/135 in BD vs. 5/107 in CG had SIJ involvement (p=0.69). Conclusion: In this study of Iranian patients, we found no significant difference in SIJ involvement between BD and controls, while the incidence of AS was higher in BD than in normal population.