o-Hydroxymandelic acid (OHMA), m-hydroxymandelic acid (MHMA) and p-hydroxymandelic acid (PHMA) were measured in the urine of 42 normotensive and 54 hypertensive patients. Patients having high urinary MHMA levels were all found to be ingesting medications containing m-synephrine (phenylephrine). These patients also had high levels of urinary m-synephrine which was excreted as the glucuronide. When patients ingesting m-synephrine were excluded from the analysis, no significant differences were observed between the two groups for the urinary excretion of OHMA, MHMA and PHMA.