The detection and diagnosis of pheochromocytoma are highly dependent on the biochemical confirmation of excessive catecholamine release by the tumor. As the reliability of baseline plasma catecholamines in the detection of pheochromocytoma is questionable, assessment of the excretion rates of catecholamines or metabolites in 24-h urine collections remains the mainstay of initial biochemical investigation. However, diagnostic difficulties can arise from incomplete collection of 24-h specimens or equivocal increases in catecholamines due to stress. To investigate the diagnostic validity of shorter collection times for the biochemical detection of this tumor, we measured the excretion of catecholamines and metabolites after sleep, a period associated with decreased sympathetic activity. Overnight catecholamines, metanephrines, and 4-hydroxy-3-methoxymandelic acid (HMMA) levels were measured in 16 patients with histologically confirmed pheochromocytomas, 166 patients with hypertension, and 24 normotensive subjects. All measurements were performed by high performance liquid chromatography with electrochemical detection. Overnight excretion of norepinephrine in the tumor group (range, 86-1552 nmol/mmol creatinine) was significantly different (P <0.001) from that in the nontumor group (14-63 nmol/mmol creatinine). Autonomous secretion of norepinephrine was evident in all urine collections, including a patient with a predominantly epinephrine-secreting tumor. Overnight normetanephrine levels displayed a similar excretion pattern (P < 0.001), whereas overnight epinephrine and metanephrine levels were normal in 10 of the 16 patients with pheochromocytoma. In contrast, HMMA excretion in overnight urine collections was highly variable, with only 6 of the 16 patients in the tumor group having consistently elevated excretion. In the other 10 patients, overnight HMMA excretion showed a high intravariability. The measurement of catecholamines and total metanephrines after sleep is a viable approach for the exclusion of pheochromocytoma, as overnight urine collections completely differentiated patients with pheochromocytoma from hypertensive patients. Compared to 24-h results, overnight urinary norepinephrine levels provided a better diagnostic sensitivity and specificity (100% sensitivity and 98% specificity compared with 88% and 82%). Sleep urine samples simplify the collection protocol while avoiding the effects of stress and exercise.