Objective: The saline infusion test (SIT) is widely used as a confirmatory test for primary aldosteronism (PA). SIT results are judged as follows: post-test aldosterone levels !50 ng/l exclude PA, whereas levels O50 ng/l confirm PA. We hypothesized that post-SIT aldosterone concentrations indicate the severity of PA and might predict outcome. Design: The study includes 256 PA patients of the German Conn's Registry who prospectively underwent SIT. The data of 126 patients with complete follow-up of 1.2G0.3 years after diagnosis were analyzed. The patients were divided into two groups with post-SIT aldosterone levels of 50-100 ng/l (group 1; nZ38) and of O100 ng/l (group 2; nZ88). Results: Patients in group 2 had a significantly shorter duration of hypertension (7.5 vs 11.7 years (median), PZ0.014), higher systolic blood pressure (BP; 151G16 vs 143G17 mmHg, PZ0.036), lower serum potassium (3.3G0.6 vs 3.5G0.4 mmol/l, PZ0.006), higher 24-h urine protein excretion (7.4 vs 5.4 mg/dl (median), PZ0.012), and were more often female (PZ0.038). They showed more often unilateral disease (P!0.005) with larger tumors (14G10 vs 7G10 mm, PZ0.021), underwent more often adrenalectomy (75% vs 37%, P!0.005), required a lower number of antihypertensive drugs after adrenalectomy (1.2G1.2 vs 2.5G1.4, PZ0.001), had a faster normalization of urinary protein excretion (with medical treatment PZ0.049; with Adx P!0.005) at follow-up, and more frequently underlying well-characterized mutation (PZ0.047). Conclusions: PA patients with post-SIT aldosterone levels of O100 ng/l have a more rapid development of PA caused more frequently by unilateral disease with larger aldosterone-producing adenomas. However, this group of patients may have a significantly better outcome following specific treatment.