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We report a case of primary intramedullary infantile fibrosarcoma (IFS) in a 4-week-old infant with rapidly progressive cervical myelopathy. Histology showed a spindle cell tumour with diffuse S100 and pan-TRK expression. RNA sequencing identified an RBPMS::NTRK3 fusion. RNA-based clustering and DNA methylation profiling classified the tumour as IFS. Treatment with the TRK inhibitor, larotrectinib, achieved marked clinical and radiological recovery within 1 month, expanding the spectrum of NTRK-rearranged sarcomas and confirming TRK inhibition's efficacy in primary central nervous system IFS.