Lack of full extension of the elbow is a common abnormality in patients with achondroplasia. We studied 23 patients (41 elbows) clinically and radiologically. Extension of the elbow was assessed clinically and the angle of posterior bowing of the distal humerus was measured from lateral radiographs. There was limited extension of the elbow in 28 (68.3%) and the mean loss of extension was 13.1 degrees. Posterior bowing of the humerus was seen in all elbows with a mean angle of 17.0 degrees. There was a positive correlation between these two measurements. Posterior bowing greater than 20 degrees caused a loss of full elbow extension. Posterior dislocation of the radial head was seen in nine elbows (22.0%). The mean loss of extension of the elbows was 28.7 degrees which was significantly greater than that of these elbows in which the head was not dislocated (8.7 degrees), although posterior bowing was not significantly different between these two groups (19.3 degrees and 16.3 degrees). Posterior bowing of the distal humerus is a principal cause of loss of extension of the elbow. Posterior dislocation of the radial head causes further limitation of movement in the more severely affected joints.