Several causes for the development of a juxtafacet cyst (JFC) of the spine have been discussed, with a focus on instability with overload of the facet joints. In the thoracic spine, JFC is a very rare space-occupying lesion, which can lead to spinal canal stenosis with pain, myelopathic signs and neurological deficits. We report a case of a 70-year-old woman who had a posterolateral fusion of L2 to S1 and, six weeks later a L1 compression fracture. In the following period a thoracolumbar kyphosis has developed. Fourteen months after the fusion procedure a left-sided JFC T11/12 was identified on MRI and CT scans which led to myelopathic symptoms. This was not seen on former MRI scans. After surgical removal of the JFC the myelopathic symptoms were clearly reduced and the woman became pain free. This case report gives a strong support to the hypothesis that spinal instability can lead to overload of the facet joints and result in JFC.