In two 3-year-old infants, a girl and a boy, systemic juvenile idiopathic arthritis was suspected because of daily fever peaks, signs of polyarthritis and general malaise. Drug treatment was unsuccessful, and after extensive laboratory investigation acute lymphoblastic leukaemia (ALL) was diagnosed and treated adequately. ALL is the most common malignancy in childhood. About one-third of the patients present with joint or bone pain and fever. In this group of children, it can be difficult to identify ALL because it may mimic the clinical picture of systemic juvenile idiopathic arthritis and because of the possibility of a normal blood count at presentation. ALL should always be considered in the differential diagnosis in children with musculoskeletal pain and fever, even in the face of a normal blood count. In any case, a bone-marrow examination should be done before steroid treatment is given.