Since the relationship between vacuolar myelopathy (VM) and HIV infection of the spinal cord is unclear, we studied 74 cases of VM in AIDS patients for associated spinal cord and brain diseases, the presence of HIV antigens (n=54) and amplified DNA (n=7) and serum levels of vitamin B,2 and folic acid (n=6). Necrosis of the medial F. gracilis was present in 14%, HIV myelitis in 9%, cytomegalovirus myelitis in 3% and microglial nodules in 16%. HIV-immunoreactive macrophages were encountered in 6% of VM cords; they were in regions of gracile tract necrosis in two and in monocytes and multinucleated cells in spinal gray matter in one. Amplified HIV DNA was detected in 50% of cords and 70% of dorsal root ganglia (DRG) of cases with moderate or severe VM and 63% of cords and 75% of DRG in AIDS cases without VM. Six of the eight spinal cords with HIV DNA contained macrophages whereas all cords without HIV DNA (n=6) were normal or contained only rare macrophages. There was no association between VM and HIV myelitis or opportunistic infections of the CNS, but there was a close association with HIV encephalitis which occurred in 80% of severe VM, 33% of moderate VM and 17% of mild VM. Vitamin B,2 and folic acid levels were normal or elevated in all six AIDS with VM as well as in 12 AIDS without VM; all had received prophylactic vitamin therapy prior to death. The results show a good correlation between macrophage infiltration and HIV antigens and DNA but little association between VM and HIV myelitis, HIV antigens or HIV DNA. In addition, HIV encephalitis closely correlated with severe VM. The association with HIV encephalitis may be fortuitous since both diseases usually occur during the late stages of AIDS or may indicate an indirect effect of HIV encephalitis upon the spinal cord.