Retroperitoneal sarcomas constitute a heterogenous group of neoplasms and often pose a diagnostic challenge. Leiomyosarcomas are a common subgroup, however improved diagnostic techniques, in particular immunohistoichemistry, electron microscopy and cytogenetics, have provided a new insight into tumours in which the lines of histogenesis or differentiation were previously unknown or only speculative. We present a case of a retroperitoneal sarcoma in a 57 year old male patient, which after complete work up was diagnosed as inflammatory leiomyosarcoma. There was a focus of heterologous bone formation also in our case, which has not been reported till date. The documentation of this entity is not only very limited in literature, but is also important as the prognosis of the completely resected tumour is better than other variants in this group.