Objectives: To define the demographic, clinical and genetic features of familial Mediterranean fever (FMF) patients with early disease onset and to compare them with late-onset FMF patients. Methods: Patients were divided into two groups according to the age of disease onset: group 1 includes the patients who had their first attack <= 3 years of age; group 2 consisted of patients who had their first attack >3 years of age. Furthermore, we compared the proportion of patients fulfilling the three diagnostic criteria among two groups. Results: Of 1687 patients, 761 had first FMF attack at <= 3 years of age while 926 patients presented with their first manifestation of FMF at >3 years. Delay in diagnosis, fever and peritonitis were significantly higher in group 1. Frequency of arthritis, erysipelas-like erythema, non-nephrotic proteinuria, incomplete attacks, chronic arthritis, arthralgia and mean colchicine dose were significantly higher in group 2. Mean Pras score was higher and the presence of M694V mutation was more frequent in group 1. The percentage of children diagnosed according to Tel-Hashomer and pediatric criteria was significantly higher in group 1 than group 2. However, both groups meet Livneh criteria similarly. Conclusion: Although patients with early disease onset seem to have more severe disease course, they are more likely to have a delay in diagnosis. To avoid the diagnostic delay, clinicians should be aware of the findings of FMF in early age.