The diagnosis of Cushing's syndrome is made on the basis of clinical assessment and dynamic hormone testing. The availability of new diagnostic and imaging techniques has improved the sensitivity of nricroadenoma detection. A positive MRI is useful, but MR images show no or equivocal evidence of tumor in 30% of pathologically proven cases.1 The false positive rate is up to 28%; therefore radiologic investigation must be used with hormone testing for tumor localization. Inferior petrosal sinus sampling clearly improves the certainty of diagnosis, and such data are useful in planning surgical therapy. In a small number of cases, however, tumor is found at operation on the side opposite to that predicted by the catheterization; it is important, therefore that both sides of the gland be explored. Nonetheless, we believe that an adequate diagnostic evaluation requires high field strength MRI as well as inferior petrosal sinus sampling in all cases of suspected microadenomas.