In seven myelomeningocele patients, six were operated upon while in one severe case no surgery was performed. The posterior fossa areas were measured and expressed by the posterior fossa ratio method. In two shunted patients both ratios were primarily below the lowest normal limit, but following shunt operation the ratios become normal. In five patients who were not shunted one only showed a normal ratio, two showed borderline values and in a fourth instance the ratio normalized but before two years had passed. The fifth case remained unoperated and therefore the natural course could be followed. In this case the decrease in the posterior fossa ratio explained the progress of hydrocephalus.