We present a case report of a ten-month-old boy, who was referred due to deviating growth and recurrent respiratory tract infections. Computed tomography of the thorax showed two large, cystic components in the mediastinum. He underwent surgical removal of the cysts. Pathological examination confirmed the diagnosis lymphatic malformation. This case illustrates that intrathoracic tumors such as lymphatic malformations, although rare, should be considered in children with deviating growth curves and respiratory problems. Rather than continuous symptomatic treatment children with symptoms early in life should be prompted in further investigations.