If the cerebrospinal fluid (CSF) is considered to be all the fluid (liquid), other than blood or the derivatives of its breakdown, that is normally contained within the brain, its cavities, and its spaces, this could be regarded as "brain fluid" in its most elemental form. "Pathological increases in intracranial CSF volume, independent of hydrostatic or barometric pressure", then, could be considered a definition of hydrocephalus. The observation of significant episodic variation in intracranial pressure (ICP) suggests the necessity of substituting the concept of "time-related pressure variations" for the older one of "level of pressure" in patients with defective ICP control mechanisms. It has been assumed that the subarachnoid channels are the first CSF compartment to dilate in response to the hydrocephalic process, reducing the CSF pressure and thereby establishing an equilibrium. When the equilibrium is disturbed, with progressive dilation of the subarachnoid channels, the increase in CSF pressure is transmitted to the ventricular system, resulting in its dilation (extraprenchymal hydrocephalus). Progressive ventricular dilation causes cerebral edema (intraparenchymal hydrocephalus) and obliterates the subarachnoid spaces as the hemispheres are compressed against the dura, resulting in apparent "internal hydrocephalus" in the absence of "external hydrocephalus". Thus, subarachnoid space or ventricular dilation occur as a result of intermittent increases in extraparenchymal CSF volume: the primary pressure force emanating from the subarachnoid and subdural spaces and from the intraventricular compartment. Hydrocephalus, therefore, may be present in a child who does not yet have dilated ventricles but in whom both CSF volume and pressure are increased. Thus, it becomes obvious that the term internal hydrocephalus is of little significance, since increases in intraparenchymal fluid--cerebral edema--cause the same volumetric changes as increases in intraventricular fluid volume. I suggest that hydrocephalus is a pathologic increase in intracranial CSF ("brain fluid") volume, whether intra- or extraparenchymal, independent of hydrostatic or barometric pressure. It may be classified as (1) intraparenchymal (cerebral edema) and (2) extraparenchymal, with the extraparenchymal types subclassified into subarachnoid, cisternal, and intraventricular forms.