Lipoma arborescens (LA) is a rare intraarticular lesion of unknown etiology, mostly described in adult patients. Unilateral knee joint is the most frequent site of involvement. LA may be associated with osteoarthritis in some patients. Magnetic resonance (MR) imaging findings are extremely characteristic in the diagnosis. We report herein a 15-year-old female patient with LA; who has atrial septal defect, and who presented with pain and swelling of bilateral knee joints. MR imaging showed characteristic signs of LA and mild osteoarthritis. In childhood patients with mono/oligoarticular effusion and synovial hypertrophy without systemic involvement; LA should be considered in differential diagnosis. MR imaging is a sensitive modality in the diagnosis of LA.