Intraspinal synovial cysts are uncommon entities associated with facet joint degeneration. Clinically the most frequent presentation is with intermittent lower back pain without motor or sensory impairment, although more classical radicular symptoms and an acute exacerbation of symptoms may occur due to intracystic haemorrhage. Previous reports include MR images from only two cases of pathologically proven haemorrhagic cysts. We report a case of acute radiculopathy shown on MR to be due to a cystic lesion adjacent to the right L4/5 facet joint and which was surgically proven to be a haemorrhagic synovial cyst. The literature on this condition is reviewed.