Objective：To assess the risk for hyperkalemia caused by treatment with angiotensin Ⅱ Type 1 receptor blockers （ARB） in clinical practice with Japanese medical database.Design：A cohort study in patients treated with ARB alone and those treated with calcium channel blockers （CCB） alone as control.Methods：The Diagnosis Procedure Combination （DPC） database provided by Medical Data Vision Co., Ltd. was used to identify patients who received a diagnosis of hypertension (ICD-10 codes, I10 to I15) and were treated with ARB or CCB from April 2008 to June 2017. A logistic regression model was applied to estimate adjusted odds ratios （OR） and their 95％ confidence intervals （CI） in these patients. The outcome in the logistic model was hyperkalemia （serum potassium≧5.5 mEq/L） and the covariates were sex, age, renal insufficiency, hepatic insufficiency, and baseline serum potassium levels. And, subgroup analysis was also performed in patients with and without renal insufficiency.Results：The incidence of hyperkalemia （per 1000 person-years） with ARB was 39.4 and that with CCB was 32.6. And, median periods from the index date to the date of occurrence of hyperkalemia for both exposure and control groups were 36 days （Min-Max：12-85） and 51.5 days（Min-Max：8-88）respectively. However, treatment with ARB was not associated with occurrence of hyperkalemia （OR 1.26, 95％CI： 0.58-2.75). The risk for hyperkalemia among those with renal insufficiency was higher （OR 3.31, 95％CI： 1.39-7.88）and as baseline serum potassium increased, the risk increased as well （OR 9.20, 95％CI： 3.52-24.10). And, the subgroup analysis also showed that rare occurrence of hyperkalemia by ARB and elevation risk for hyperkalemia by baseline serum potassium.Conclusion：The clinical data showed rare occurrence of hyperkalemia caused by ARB, indicating that renal insufficiency and baseline serum potassium levels affected the onset of the disease in clinical practice. Previous studies also reported the effects of renal insufficiency and other factors on the onset of hyperkalemia. ARB should be prescribed carefully in patients with these factors, as is conventionally done.