BACKGROUND Spinal lesions in the acquired immunodeficiency syndrome (AIDS) occur in 22% of all neurological complications, although their occurrence as the first sign of the disease is very uncommon. First place amongst the myelopathies described as associated with HIV is taken by vacuolar myelopathy, followed by the myelites. METHODS We studied a 65 year old homosexual man who had previously been free of symptoms and signs of HIV infection. He was seen for progressive paraparesia which had begun some days previously, followed by altered sense of position and of vibration. Complementary tests showed that the patient was a carrier of HIV antibodies in high titre (P-24) with a high viral load of 907 x 1,000 copies RNA/ml. Testing with evoked potentials showed changes compatible with involvement of the sensory pathways of the spinal cord. After treatment with dexamethasone there was obvious improvement with partial recovery of his symptoms. CONCLUSIONS The neurological condition described was compatible with vacuolar type myelopathy due to HIV, appearing in the initial stages of the illness and coinciding with a marked increase in the viraemia. Although necropsy is necessary for confirmation of the diagnosis, more and more evidence is being found on which to base clinical suspicion. We discuss various hypotheses regarding possible pathogenic mechanisms arising in these circumstances.