The grid test for schizophrenic thought disorder was administered to a sample of 149 psychiatric patients. This sample was drawn from a population with a known base rate of schizophrenia and an estimated base rate of thought disorder, thus allowing for the calculation of population specific cutting scores for the grid test.The grid test, using these data along with others from previous grid test studies, was then analysed for diagnostic efficiency using conditional probability formulae.The conclusion reached is that the grid test for schizophrenic thought disorder is unsatisfactory as a diagnostic instrument.