['The aim of this study was to assess the effects of hinged ankle foot orthoses (AFO) on the metabolic and cardiopulmonary cost of walking and gross motor skills of children with cerebral palsy (CP).', 'Ten habitual users of hinged AFO with spastic diplegic CP (9.01 yr +/- 2.10) participated in the study. Expired gas and heart rate (HR) were measured during sitting and with AFO on and off during steady state treadmill walking at three speeds: 3 km.h(-1), comfortable walking speed (CWS), and 90% of their fastest walking speed (FWS). Comfortable and fastest ground walking speed and Gross Motor Function Measure scores were also assessed with AFO on and off and analyzed with ANOVA. Because not all children could walk at all speeds on the treadmill, an ANOVA was performed on data for children who walked at 3 km.h(-1) and CWS (N = 8 for HR; N = 9 for pulmonary ventilation and metabolic variables) and a t-test on data at 90% of FWS (N = 9 for HR; N = 8 for pulmonary ventilation and metabolic variables).', 'When children wore their AFO net oxygen uptake (L.min(-1), absolute--sitting values) was significantly (P < 0.05) reduced by 8.9% at 3 km.h(-1) and by 5.9% at 90% of FWS. Net pulmonary ventilation (L.min(-1)) was significantly (P < 0.05) lower with AFO on by 10.3% but only at 3 km.h(-1). AFO did not affect net HR (beats.min(-1)) nor the respiratory exchange ratio at any speed, nor any physiologic variable at CWS, nor gross motor skills.'] Does use of orthoses lower the O ( 2 ) cost of walking in children with spastic cerebral palsy?