From a series of 681 patients with lumbar disc disease treated between 1966 and 1978, 17 patients required surgical lysis of lumbar adhesive arachnoiditis, 8 having initially been operated upon by another surgeon. All patients had severe pain as a predominant feature, with pain being bilateral in 9 patients. Pain was the only major symptom in 3; the other 14 exhibited varying combinations of progressive neurological dysfunction. Three patients developed late symptoms after trauma, 8 to 21 years after back surgery. At operation, multisegmental arachnoiditis was found in 5 patients and anular or subtotal adhesions were found in 12. Complete lysis could not be obtained in 4 patients. Fourteen patients were treated with steroids at the time of operation. Follow-up after lysis was less than 1 year for 5 patients but averaged 4.8 years for the remaining 12. During the 1st year after operation, 76% experienced improvement in pain (35%, good to excellent), and 71% experienced improvement in neurological status. Follow-up after at least 1 year revealed 50% still enjoying pain relief (25%, good to excellent) and 45% experiencing neurological improvement. Pain relief persisted in 4 of 5 patients followed 5 years or more. The etiological role of myelography and lumbar disc surgery in arachnoiditis has probably been over-rated. Arachnoiditis may be symptomatic or asymptomatic and may mask other, treatable lumbar lesions. More frequent intradural exploration for discrepancies between operative and myelographic findings might reveal, and benefit, more cases of spontaneous arachnoiditis mimicking lumbar disc disease.