OBJECTIVE To evaluate diagnostic criteria in primary aldosteronism, we studied the sensitivity and specificity of potassium, renin, aldosterone and the renin/aldosterone ratio in 60 patients undergoing surgery for Conn's adenoma, 50 patients with primary hypertension and 49 normal controls. We also searched for a relationship between these parameters and the blood pressure outcome of surgery. METHODS The diagnostic value of the tests was quantified using the Youden index after adjustment for receiver operating characteristic (ROC) thresholds. RESULTS Potassium level in patients was lower than in controls, but in 22%, kaliemia was > or = 3.5 mmol/l and the threshold giving the best Youden index (0.93) was 3.9 mmol/l. The diagnostic power of active renin was low (Youden index 0.28), but the Youden indexes for aldosterone level and the aldosterone/renin ratio in supine position were 0.68 and 0.66 respectively. After a mean follow-up of 8.7 months after surgery, 70% of the patients had normal or improved blood pressure levels. None of the biological parameters evaluated was associated with blood pressure outcome, but age > 55 years was related to unfavorable outcome (sensitivity and specificity 80 and 60%). CONCLUSIONS The threshold level requiring a search for an adenoma should be raised. When the potassium level is < or = 3.9 mmol/l the aldosterone/renin ratio should be measured in supine position since it evaluates the dissociation between renin and aldosterone seen in primary hyperaldosteronism. The effect of age on the surgical result emphasizes the importance of early diagnosis.