OBJECTIVES--To assess the incidence of post-traumatic syringomyelia (PTS), to correlate the presence of PTS with its most common signs and symptoms, and to compare results from the Swiss Paraplegic Centre with those reported in the medical literature. METHODS--A total of 449 recent traumatic paraplegic and tetraplegic patients admitted to the Swiss Paraplegic Centre in Zurich between 1 January 1987 and 31 December 1993 were prospectively analysed. Yearly clinical tests with conventional radiographs and additional T1 and T2 weighted images were performed as soon as PTS was diagnosed. RESULTS--Of these 449 patients 20 patients displayed symptoms of PTS (4.45%). Ten non-operated patients remained clinically stable (average time: 37 months). Ten worsened--three refused operation, seven were operated on. Mean worsening time was 97 months. Deterioration was closely related to the enlargement of the cyst whereas in operated patients neurological improvement or stabilisation correlated with collapse of the cyst. CONCLUSIONS--Delay between appearance of the first symptoms of PTS and deterioration making surgery necessary may be long (mean five years in the seven operated patients) underlining the need for regular tests. "Slosh" and "suck" mechanisms could explain cyst enlargement as surgical realignment of the spine resulted in a complete cyst collapse in two of the operated patients (normalisation of CSF flow? ). Cord compression, tense syrinx at the fracture site, and kyphosis seemed to be closely linked to the enlargement of the cyst with subsequent further neurological deterioration.