Steroid injection of the sacroiliac joints is widely used for the management of active sacroiliitis in patients with spondyloarthritis (SpA). The aims of this study were to prospectively investigate the effectiveness of corticosteroid injection of the sacroiliac joints and to identify factors determining positive and negative outcome. Twenty-nine patients (18 female, 11 male) suffering from sacroiliitis were treated by computed tomography (CT)-guided administration of 40 or 60 mg triamcinolone acetonide per joint. Clinical outcome of the intervention was determined using a visual analog scale from days 1 to 7 as well as after 1, 3, and 6 months. Patients were grouped according to intra-articular (n = 22) and peri-articular positions (n = 7) of the tip of the puncture needle. In patients with intra-articular needle position (76 %), the mean pain score decreased significantly from 7.3 (±1.9) to 3.9 (±3.4) at day 7 and to 3.9 (±3.4) after 1 month, 4.2 (±3.3) after 3 months, and 5.2 (±2.9) after 6 months. The group with peri-articular needle position (24 %) did not show a statistically significant pain reduction throughout the entire 6-month follow-up period. A substantial reduction of inflammatory back pain (reduction of at least 4 visual analog scale (VAS) points) was reported by 55 % of patients after 3 months and 45 % of patients after 6 months. Our results demonstrate that intra-articular CT-guided steroid instillation can achieve sufficient pain and symptom control for 6 months in patients suffering from active sacroiliitis. It is therefore recommended to perform this intervention under appropriate image guidance to ensure proper positioning of the needle tip.