Background: Acne vulgaris is a common complication that patients referred to dermatologist, also there are medications to manage of this such as isotretinoin, and incidence of low back pain in these patients is more than general population, so in this study we aimed to evaluate of Incidence of low back pain and sacroiliitis in military families with acne vulgaris under isotretinoin therapy. Methods: This cross-sectional study was performed on 113 acne vulgaris patients treated with isotretinoin (at least 3 months) referred to dermatology clinic of Imam Reza Hospital of Army Medical Sciences, Tehran, Iran during 2018 to 2019. All data of patients such as dosage and duration of medication, severity of acne, incidence of low back pain, sacroiliitis and inflammatory back pain and lab data were recorded. Results: 54% of patients had lethargy, 41.6% had myalgia and 46.9% had low back pain. Of the patients with low back pain, 54.7% had inflammatory low back pain and 45.3% had mechanical low back pain. 29 cases had inflammatory low back pain, 5 cases had sacroiliitis and one case had positive RF, the means of ESR, CRP and ANA were 19.01 ± 12.74 Mm/hr, 3.43 ± 1.27 Mg/litr and 1.31 ± 0.90, respectively. Conclusion: In patients with acne vulgaris under isotretinoin therapy much attention must be given regarding to sacroiliitis and rheumatoid diseases especially when symptoms such as low back pain are present.