Posttraumatic spinal cord cavitation (syrinx) and progressive myelopathy associated with a posttraumatic syrinx (PTS) have been recognized for many years. Predisposing factors for PTS have not been well defined. All symptomatic PTS patients (n = 58) treated at the University of Toronto neurosurgical units over 30 years were analysed retrospectively to discover risk factors for the development of PTS. There were 46 males and 12 females (mean age 42.2 years); 28 patients had cervical (CSCI), and 30 had thoracic and lumbar spinal cord injuries (TSCI). Neurological deficit was complete in 40 (69%) compared with 43% complete injuries in the concurrent spinal and injury (SCI) population in general (p = 0.0003). Onset of PTS was earlier with: increasing age (p < 0.02), cervical and thoracic levels of injury (p < 0.007 and p < 0.02, respectively, compared with lumbar injuries), dislocated fractures (p < 0.003) and spinal surgery (p < 0.001) and, marginally, with complete SCI (p = 0.06). Mean time to onset of PTS appears to have decreased in recent years (p < 0.0001). PTS follows complete spinal cord injury (SCI) more often than incomplete and is frequently associated with arachnoiditis. Onset is earlier with increasing age, cervical and thoracic levels compared with lumbar, displaced fractures and spinal instrumentation without decompression. A group of SCI patients who are at greater risk of developing early PTS is thus identified, facilitating cost-effective follow-up.