Background The pathophysiological origin of juvenile idiopathic arthritis (JIA) is largely unknown. However, individuals with presumably pathogenic mutations in FAMIN have been reported, associating this gene with a rare subtype of this disorder. FAMIN, that is formerly also referred to as LACC1 or C13orf31, has recently been shown to play a crucial role in immune-metabolic functions and is involved in regulation of inflammasome activation and promotion of ROS production. Case presentation We describe two siblings with severe familial forms of juvenile arthritis in which whole-exome-sequencing revealed a novel homozygous frameshift mutation (NM_153218.2:c.827delC¸. p.(T276fs*2) in FAMIN. Conclusions The observation of a new deleterious mutation adds further evidence that pathogenic mutations in FAMIN are causal for a monogenic form of JIA. Furthermore the associated phenotype is not restricted to systemic JIA, but can also be found in other forms of familial juvenile arthritis.