Sinding-Larsen-Johansson syndrome is an osteochondrosis affecting the inferior pole of the patella. Most cases can be easily diagnosed with adequate clinical history, physical examination, and proper imaging, including conventional radiography, ultrasound, and magnetic resonance imaging. Differentiating this condition from patellar sleeve avulsion fractures is important, since treatment is frequently surgical in the latter. Overlap between these two conditions can also occur. We present a case of an 11-year-old boy, with Sinding-Larsen-Johansson syndrome on both knees and a minimally displaced acute patellar avulsion sleeve fracture of the left knee, which was treated conservatively.