A 2-year-old Collie was examined after it developed acute lameness in the right hind limb. Palpation of the right stifle elicited signs of pain, but a cranial drawer sign could not be elicited during palpation of the joint. Radiography revealed mineralized opacities in the craniolateral and caudal portions of the right stifle joint. Exploratory arthrotomy of the right stifle joint revealed a firm mass in the cranial horn of the lateral meniscus, which was characterized as osseous metaplasia in the cranial and caudal menisceal horns on histologic examination. Other abnormalities were not identified during surgery. Results of histologic examination, radiologic evidence, and observations during exploratory surgery supported a diagnosis of primary lateral meniscal ossification of the right stifle joint. Radiographs were obtained of the contralateral stifle joint when the dog was clinically normal (5 weeks after arthrotomy), and mineralized opacities were found in the lateral meniscus of this limb as well. Clinical signs may not be associated with primary intrameniscal ossification initially; type and location of ossification may affect likelihood and severity of clinical signs and secondary joint damage. Initial treatment for menisceal ossification is conservative (anti-inflammatory medications) unless there is a concurrent pathologic process that requires surgical repair.