MEP targeting during BoNT-A injections has been demonstrated to improve outcome. Two injection techniques of the psoas muscle--proximal MEP targeting versus a widely used more distal injection technique--are compared using muscle volume assessment by digital MRI segmentation as outcome measure. Method: 7 spastic diplegic children received injections in both psoas muscles: two different injection techniques randomly in 5 patients, in 2 patients bilateral MEP targeting. MRI images of the psoas were taken before, after 2 months and in 3 patients after 6 months. Results: Average post injection volume (in relation to pre-injection volume) for the MEP targeted muscles (9) is 79.5% versus 107.8% in the 5 distal injected psoas muscles (p = 0.0033). In all 5 asymmetric injected patients the MEP targeted psoas had a larger volume reduction than the more distal injected psoas muscle. This atrophy remains even 6 months after the injection. This is the first study were a longitudinal follow-up by MRI demonstrates muscle atrophy after BoNT-A in children with CP. Injections in the MEP zone of the muscle, which is the more proximal part of the psoas muscle, cause atrophy in contrary to more distal injections were this atrophy is not observed. (Contains 4 figures.)