In 110 patients the diagnosis of pheochromocytoma was established by biochemical tests. These patients displayed various patterns of urinary and plasma catecholamines and their metabolites. The estimation of urinary methoxycatecholamines proved the most useful screening procedure. In patients with equivocal biochemical results the clonidine suppression test appeared to be a useful diagnostic tool. Clinical analysis displayed a bizzare course and unusual symptoms and signs in some patients. Computerized tomography and adrenal scintigraphic imaging with <sup>131</sup>I-meta-iodobenzylguanidine were found to be the most reliable non-invasive methods for localization of the tumor.