Foci of high signal in the cerebral white matter are common incidental findings on MR images of the brain of control subjects or patients with a variety of diseases. Although the number of foci has been reported to correlate with age and several risk factors, the degree of observer variability in quantifying foci has not been reported. We used kappa statistics to determine radiologists' agreement in counting high-signal-intensity foci on MR images obtained in healthy volunteers and in patients with hypertension. Before interpreting the images, one pair of radiologists studied 30 routine MR images and reached consensus on differentiating high-signal foci from other foci of high intensity caused by normal structures (e.g., deep gyri or Virchow-Robin spaces). These two observers than independently determined the number of foci in the study group. Using their own criteria, other radiologists independently counted the foci. Agreement between observers was determined with the kappa statistic. The results showed fair agreement between the radiologists who first reached a consensus in counting foci of hyperintensity and poor agreement between the other observers. We conclude that in order to compare the frequency of foci of hyperintensity in different groups of patients, observer variability must be controlled. Studies without proper control subjects may lead to incorrect conclusions regarding the correlation of focal hyperintensities and various risk factors.