Hip abduction orthoses are commonly used as adjuncts to create a symmetrical and stable sitting position for children with spastic diplegia. This study investigated how activity in m. Adductor longus, m. Rectus femoris, m. Gastrocnemius and m. Erector spinae was affected while sitting in a ''functional sitting position'' (FSP) performing five upper extremity tasks under four test conditions, one without hip orthosis and three with the orthosis set at three different spring lengths. The experimental group consisted of four children with spastic diplegia (mean age 4 years), while five healthy children (mean age 7 years) served as controls. The initial hip abduction angle adopted spontaneously by each child was measured by a goniometer. Muscle activity was measured by surface electromyography (EMG) during three minutes under each test condition. The children in the experimental group were also assessed from videofilms with the Sitting Assessment Scale (SAS) and the SAS scores were compared to the EMG results. The results of the study failed to demonstrate significant difference in EMG voltage under different sitting conditions, without or with hip abduction orthosis adjusted to varying hip abduction positions. This may imply that if a functional sitting position is assured, further addition of an orthosis is not needed. Follow-up study on a larger group of children with CP may be advisable