Purpose: Undifferentiated round cell sarcomas are a rare form of highly aggressive bone and soft tissue tumours. There are very few reported cases and no established treatment guidelines. We describe two cases of initially misdiagnosed neonatal sarcomas. The first case is of a female neonate born with a large left ulcerating flank mass, not antenatally diagnosed. The mass was diagnosed and treated initially as kaposiform haemangioendothelioma. Initial surgical excision of two components measured 11x8x2cm over the left flank and a 5x5x2cm anterior extension, followed by re-excision for clearance of involved margins. Histology diagnosed round cell sarcoma with BCOR-CCNB3 fusion. They were treated with chemotherapy on an Ewing-type protocol with no recurrence. The second case is of male twin pregnancy born via emergency caesarean section at 35 weeks gestation with a left flank lesion noted at time of birth, not antenatally diagnosed. Initially treated as infantile haemangioma, the tumour progressed and was resected, measuring 15x13x4cm. Histology diagnosed a BCOR-associated sarcoma with YWHAE::NUTM2B fusion. Surgical clearance was not performed. The patient undergoes adjuvant chemotherapy treatment with clinical response. Conclusion: There are few reported cases of BCOR-associated undifferentiated round cell sarcoma in neonates. We describe the initial misdiagnosis of two neonatal cases as vascular malformations and subsequent management once malignancy was confirmed.