Context Fludrocortisone (FC) is the mineralocorticoid (MC) replacement treatment for patients with primary adrenal insufficiency (PAI). Objective To explore the dose of FC treatment and its relationship with glucocorticoid therapy, sodium, potassium, renin and clinical parameters. Setting Monocentric cohort. Patients Data of 193 patients with PAI (130 autoimmune) were collected during baseline (T0), intermediate (T1) and last follow-up visit (T2, respectively, after a mean of 38 and 72 months). Main outcome measure Utility of endocrine and clinical parameters to titrate FC dose. Results FC dose (50–75 μg/daily) was stable in the follow-up in half patients. The MC activity of FC was dose-dependent: we observed a reduced but significant positive linear correlation between FC dose and sodium (r = 0.132) and negative linear correlation between FC and potassium (r = − 0.162) or renin (r = − 0.131, all p < 0.01). An overall reduction in the FC dose was observed at T2 in the group with longer follow-up (> 60 months, p < 0.05). Higher doses of FC were observed in patients with low-normal renin, especially in autoimmune PAI (86 vs 65 μg/daily, p < 0.05). On the contrary, reduced sodium and increased potassium levels were observed in patients with high renin at T2. The number of cardiovascular events (15 in the whole cohort) was similar in patients sorted by renin levels or FC dose. Conclusions Renin and electrolytes can indicate the MC activity of FC treatment: they should be routinely evaluated and used to titrate its dose that can be reduced in the long-term follow-up.