Six members of a family developed a previously unreported disorder resembling chronic adhesive spinal arachnoiditis both clinically and pathologically. Affected members initially had adult onset of progressive spastic paraparesis with prominent radicular features of pain and patchy numbness. Myelography in three cases demonstrated complete or partial obstruction to flow of contrast material in the thoracic region along with multiple filling defects and fragmentation of contrast material. Exploratory surgery in two patients revealed, in both instances, a band-like fibrous thickening of spinal arachnoid, with or without cyst and bone formation encircling the spinal cord at the level of the myelographic block. The primary event appears to be a genetically determined (autosomal dominant) collagenous thickening of spinal arachnoid membrane with secondary ischemic radiculomyelopathy.