Hyperkalemia is an important complication of adrenalectomy for patients with primary aldosteronism (PA). The frequency of hyperkalemia after medication using mineralocorticoid receptor antagonists (MRAs) for PA is unclear. The aim of this study is to investigate the frequency and the risk factors of hyperkalemia after surgery and medication for PA. The data of 376 patients with PA registered in a multicentre-collaborative study in Japan, including surgically treated patients (group A; n = 142) and medically treated patients with MRAs (group B; n = 234) were studied. The prevalence of hyperkalemic patients (serum potassium > 5.0 mEq l-1) after treatment was higher in group A than group B (9.9 vs 3.8%, P < 0.01). At diagnosis, the hyperkalemic patients were older and had a poorer renal function than the non-hyperkalemic patients in both groups (P < 0.05). The hyperkalemic patients had severer PA in group A and milder PA in group B. The independent risk factor by a logistic regression analysis was only age in both groups. After treatment, the percentages of patients withdrawing antihypertensive drugs and the normalization of aldosterone renin ratio were not different between hyperkalemic and non-hyperkalemic patients in group A. The type and dose of MRAs and the combination of other antihypertensive drugs were not different between hyperkalemic and non-hyperkalemic patients in group B. In conclusion, the potential occurrence of hyperkalemia should be considered after medical as well as surgical treatment for PA, especially in patients with older age (> 60 years) and impaired renal function (estimated glomerular filtration rate < 70 ml min-1 per 1.73 m2) at diagnosis.