PURPOSE: Longitudinal fibular deficiency (LFD) is the most common congenital long bone deficiency. This study aimed to objectively assess the physical performance of children and adolescents with LFD compared with unaffected peers, and to examine trends over age for subgroups of the LFD population. METHODS: Differences between children with LFD and unaffected peers were examined with hand-held dynamometry for lower-limb muscle strength, Six-Minute Walk Test, Timed up and down stairs test, Star Excursion Balance Test, and Standing long jump. RESULTS: Thirty-nine children with LFD and 284 unaffected peers participated. Children with LFD performed at a lower level than their unaffected peers, on all measures of physical performance (mean 2.1 z-scores lower, all p < 0.01), except in long jump (p = 0.27). When comparing the performance of children with LFD to their unaffected peers across four age groups, there was a significant between-groups difference on all strength measures, and on the Six-Minute Walk distance, between children with and without LFD. These differences were smallest in young children (3-6 years) and largest in the older children (15-18 years) (all p < 0.01). Children with no lengthening surgery performed better on the Six-Minute Walk Test, covering a greater distance during the test, than those who had surgery (mean difference 83 metres, p < 0.01). There were no significant differences between children who had or had not undergone an amputation. CONCLUSIONS: Children with LFD performed at a significantly lower level than unaffected peers on all measures of physical performance other than jumping. The largest differences were in older children. This paper provides baseline functional data for future interventions in LFD. LEVEL OF EVIDENCE: Cross-sectional study.Implications for RehabilitationThis paper provides the first baseline functional data using validated objective measures on a consecutive cohort of children and adolescents with longitudinal fibular deficiency.Children with LFD performed significantly worse than their unaffected peers on all measures of physical performance other than jumping, with children falling further behind their peers as they age.Children who undergo an amputation typically have the most severe anatomical presentation and yet perform at an equivalent functional level.This paper identifies multiple modifiable impairments that represent potential opportunities for rehabilitation professionals to target with conservative treatment options to improve functional performance.