A child with Rubinstein-Taybi syndrome was diagnosed at birth with contraction of the left knee in flexion. A sonography of both knees, performed at 4 months of age, resulted in diagnosing a laterally displaced, hypoplastic left patella. Surgery was performed at 8 months of age as described by Stanisavljevic, which resulted in correct alignment of the patella. Follow-up examination at 2.5 years of age showed a contracture in flexion of 10 degrees , passively redressible with application of gentle force and clinically correct alignment of the patella. Sonography showed the left patella to be similar in size, shape, and position to the right patella. The child still did not walk because of mental retardation, but stood upright unsupported. Stanisavljevic procedure in this case gave good results. Sonography gives excellent view of the unossified patella, as well as other nearby soft tissues, and is sufficient for a correct diagnosis.