Background In a majority of patients with suspected sacroiliitis (SI) who underwent sacroiliac magnetic resonance imaging (MRI), imaging studies may be normal, may depict other causes for pain, or may show clinically irrelevant incidental findings. Purpose To determine the prevalence of possible etiologies other than SI and frequency of incidental findings demonstrated on sacroiliac MRI examinations in a cohort of patients with lower back pain and suspected SI. Material and Methods Sacroiliac MRI examinations of 1421 patients with suspected SI were retrospectively reviewed. In patients without SI findings, other potential causes for lower back pain and incidental findings were documented. Results SI was present in 535 of 1421 patients (37.6%). In 886 of the patients whose MRI studies were negative for SI, other possible causes for lower back pain or incidental findings were seen in 386 (43.5%). The most common musculoskeletal (MSK) finding was lumbosacral transitional vertebra (8.6%) followed by findings suggesting piriformis syndrome (4.2%), spondylosis (3.7%), and sacral insufficiency fractures (1.8%). The most common non-MSK findings were follicular cysts (15.3%) and uterine fibroids (4.9%). Conclusion In patients with suspected SI but negative MRI examinations for SI, some other possible causes for lower back pain and several incidental findings can be seen on imaging. The presence of these findings may explain the patient's symptoms, and awareness of these conditions may be helpful in patient management and individualizing treatment.