When administered to a 45-year-old woman who was ill with severe hypertension, combined enalapril and hydrochlorothiazide therapy resulted in acute renal failure, leading to a successful search for renal artery stenosis. Acute renal failure as an adverse reaction to enalapril therapy occurs in approximately 20% of patients with secondary hypertension from renovascular disease. On the other hand, in a group of essential hypertensives this occurs at a rate of approximately 0.2%. Thus, a likelihood ratio of 100 is produced, and given the prevalence of renovascular hypertension of approximately 2% in all hypertensives, a posterior probability of 0.67 can be calculated. The occurrence of this adverse effect should indicate that a diagnostic work-up to rule out renovascular hypertension in such patients should be pursued. The sensitivity and specificity of this “test” are 0.20 and 0.998, respectively. This illustrates that the mathematic aids in decision theory need not be applied solely to those more traditional aspects of diagnostic testing, but may be useful in other situations as well.