Simultaneous intraventricular and intraspinal pressure recordings in erect patients with obstructive lesions of th CSF pathways reveal differences in pressure which are frequently transitory and produced by pulsation. In non-acute cases without papilloedema but with suspected hindbrain hernia delay in equalization after pressure pulses may be demonstrated, and after Valsalva's manoeuvre differences between the head and the spine may be generated transiently and be responsible for clinical symptomatology. The particular clinical features related to hindbrain hernia are syringomyelia, cough headache, cough syncope, and lower cranial nerve signs with oscillopsia and cerebellar ataxia. Correction of the pressure dissociation is often associated with marked clinical improvement. It is suggested that this form of testing may be of relevance as an indication for operation and also for monitoring the progress of post-operative patients.