Familial Mediterranean fever (FMF) is the most common monogenic autoinflammatory disease manifesting with phenotypic heterogeneity. The phenotype-genotype correlation is not established clearly yet. Furthermore, some comorbidities such as vasculitis and inflammatory arthritis may accompany FMF. Herein, we aimed to define phenotype-genotype correlation and comorbid diseases of children with FMF. The medical records of 1687 children diagnosed and followed up as FMF were reviewed retrospectively. Disease severity was assessed by PRAS score. A total of 1687 children (841 girls, 846 boys) were involved in the study. The mean +/- standard deviation of current age, age at symptom onset, and age at diagnosis were 13.1 +/- 5.4, 5.4 +/- 4, and 8 +/- 4.2 years, respectively. Median (min-max) follow-up period was 3 (0.5-18) years. Among them, 118 (7%) patients had at least one concomitant disease and 72% of them were carrying at least one M694V mutation. Patients with a concomitant disease expressed a more severe course of disease when compared to ones without a concomitant disease (23.7% vs 8.8%, p < 0.001). Children carrying homozygous M694V mutation had significantly earlier age of disease onset and severe disease course (p < 0.001). Forty-four patients (2.6%) were colchicine resistant and most of them were carrying homozygous M694V mutation. Sixteen colchicine-resistant patients were treated with anakinra while 28 received canakinumab. Juvenile idiopathic arthritis (JIA) and immunoglobulin A vasculitis were the most commonly seen associated diseases and the patients with a concomitant disease demonstrated more severe course. This is the largest pediatric cohort studied and presented since now. We confirmed that carrying M694V mutation is associated both with a severe disease course and a predisposition to comorbidities.