The radicular cysts developed on primary teeth are infrequent compared to their evolution in permanent teeth. However, dentigerous cysts are ranked second in frequency after inflammatory cysts. They surround the crown of a tooth erupting while the radiculo-inflammatory dental cysts are hanging from the roots of the mortified teeth. This is the probable coexistence of these two entities that we have been faced in a patient aged 10 years with mixed dentition who presented a radiolucent lesion surrounding infected roots of a mandibular second primary molar in the vicinity of the tooth successor. In our presentation, we discuss problem diagnosis but also treatment. Indeed, we proceeded to avulsion of deciduous molar and we opted for a cystotomy or marsupialisation to preserve the replacement tooth.