Aim: The aim of this study was to determine the longitudinal associations among fitness components and between fitness and mobility capacity in children with cerebral palsy (CP). Method: Forty-six children (26 males, 20 females; mean age 9y 7mo [SD 1y 8mo]) with a bilateral (n=24) or a unilateral spastic CP (n=22) participated in aerobic and anaerobic fitness measurements on a cycle ergometer and isometric muscle strength tests (Gross Motor Function Classification System [GMFCS] level I [n=26], level II [n=12], level III [n=8]). Mobility capacity was assessed with the gross motor function measure (GMFM) and a walking capacity test. Associations over longitudinal measurements (three or four measurements over 1y) were determined since longitudinal data allow a more accurate estimation. The associations were determined using a mixed model with fixed effects (mobility capacity as dependent variables and fitness components as independent variables) and a random intercept. Results: In children with bilateral CP, changes in aerobic fitness were associated with changes in anaerobic fitness (p<0.001), and changes in aerobic fitness showed an association with changes in muscle strength (p<0.05). Anaerobic fitness was not associated with muscle strength. No associations between fitness components were found in unilateral CP. Anaerobic fitness and muscle strength were significant determinants for GMFM and walking capacity in bilateral but not in unilateral CP. Interpretation: The longitudinal associations between aerobic and anaerobic fitness and mobility indicate that increasing either aerobic or anaerobic fitness is associated with improvements in mobility in children with bilateral CP. While increasing anaerobic fitness might be beneficial for mobility capacity in children with bilateral CP, this is less likely for children with unilateral CP.