Clinical reports have suggested that myelography and laminectomy may produce more arachnoiditis than myelography alone. The effect of experimental lumbar myelography and laminectomy on arachnoiditis in monkeys was studied. Arachnoiditis was as severe after myelography alone as after myelography and laminectomy. Minimal arachnoiditis was found myelographically and histologically after myelography with metrizamide 300 mg l/ml, and severe arachnoiditis was found after myelography with iophendylate whether or not laminectomy was performed. Laminectomy alone produced insignificant arachnoid changes. Experimental myelography preceding laminectomy did not increase the risk of arachnoiditis.