Factors related to treatment resistance in hypertension. Hypertension which is resistant to treatment carries a relatively bad prognosis. Factors associated with treatment resistance were examined in a case-control study in a hospital hypertension clinic. Patients with resistant hypertension had more severe hypertension and more frequently had evidence of end-organ damage on presentation to the clinic. The prevalence of accelerated phase hypertension, renovascular disease and impaired renal function was also higher in these patients. Cigarette smoking, and the combination of cigarette smoking and heavy caffeine use, were greater in patients with resistant hypertension. Resistant hypertension did not appear to be associated with older age, obesity, regular alcohol use, various psychological factors or non-compliance. These findings support an aggressive investigation policy in resistant hypertension, and underline the harmful effects of cigarette smoking to hypertensive subjects. 