Aim: To explore lower leg muscle morphology and intramuscular fat content changes using magnetic resonance imaging (MRI) after 12 months of treatment‐as‐usual (TAU), after the first botulinum neurotoxin A (BoNT‐A) injection in children with cerebral palsy (CP) naive to muscle tone reduction treatment.

Method: This prospective longitudinal study included 10 children with spastic CP (five female, median age 8 years 11 months, unilateral CP/bilateral CP 7/3, Gross Motor Function Classification System levels I/II 8/2) and 10 typically developing peers (seven female, median age 9 years 6 months). Children with CP received two MRI scans (before and 12 months after the first injection), typically developing peers received one. Muscle volume (normalized by product of weight and height), fascicle length (normalized by height), pennation angle, and intramuscular fat fraction (IFF) of medial gastrocnemius, lateral gastrocnemius, soleus, and tibialis anterior were quantified.

Results: After 12 months of TAU, normalized muscle volume remained unchanged while IFF increased (medial gastrocnemius: p  = 0.008). Compared to typically developing peers, children with CP had smaller normalized muscle volume (tibialis anterior: p  = 0.027) and similar IFF at pre‐injection timepoint.

Interpretation: Although normalized muscle volume remained unchanged after the 12‐month TAU, concurrent fat infiltration was evident in medial gastrocnemius.