Objectives: The aim of this study was to demonstrate the frequency of macrophage activation syndrome (MAS) in systemic juvenile idiopathic arthritis (sJIA) cases, to compare the laboratory tests at the time of diagnosis of sJIA and MAS and to see whether sJIA cases complicated with MAS follow a more severe disease course in the long-term follow-up. Methods: Files of children with sJIA that were followed between May 2010 and September 2017 were reviewed. Results: The cohort consisted of 53 sJIA cases. Mean duration of follow-up was 39.0 +/- 24.1 months. The frequency of MAS was 33.9%. Initial laboratory tests at the time of diagnosis of sJIA were compared in between patients with MAS and without MAS. Only ferritin and fibrinogen levels showed significant differences in between the groups (p < .01). Patients who developed MAS had higher ferritin (4482 mg/dL) and lower fibrinogen (371 mg/dL) values than patients without MAS (ferritin 2060 mg/dL, fibrinogen 466 mg/dL) at the time of diagnosis of sJIA. Long-term follow-up results showed that monocyclic course was observed in 45.2%, polycyclic course in 30.1% and persistent course in 24.5% of the cases. It was seen that patients with MAS segregated equally into three groups. Conclusions: Higher ferritin and relatively lower fibrinogen levels at the time of diagnosis of sJIA may be early warning signs of an impending MAS. sJIA patients who develop MAS do not seem to warrant more guarded prognosis in the long-term follow-up.