We report two cases of diabetes mellitus treated with sacubitril/valsartan, whose urinary C-peptide dynamics exhibited significant difference. The first case is an 84-year-old Japanese man with type 2 diabetes and hypertension who exhibited significantly elevated urinary C-peptide levels during treatment with sacubitril/valsartan, an angiotensin receptor-neprilysin inhibitor (ARNi). Despite normal serum C-peptide levels, his urinary C-peptide excretion was disproportionately high, suggesting that sacubitril/valsartan may have altered C-peptide clearance through neprilysin inhibition. The discontinuation of sacubitril/valsartan demonstrated gradual decline of urinary C-peptide levels, although they remained elevated compared to serum ones. Daily longitudinal monitoring of urinary C-peptide revealed marked fluctuations of its levels, indicating a prolonged and potentially complex effect of neprilysin inhibition on C-peptide excretion. Additionally, we observed a corresponding decrease in atrial natriuretic peptide (ANP) levels after discontinuation of sacubitril/valsartan, further supporting the role of neprilysin inhibition in altering peptide metabolism. In contrast, the second case involves a 78-year-old Japanese woman with insulin-dependent type 1 diabetes and undetectable serum C-peptide levels. In her case, urinary C-peptide levels remained undetectable despite ARNi therapy. These cases highlight the need for careful clinical interpretation of urinary C-peptide levels in patients receiving neprilysin inhibitors. When evaluating pancreatic β-cell function using urinary C-peptide levels under ARNi therapy, it is crucial to consider extended monitoring of urinary C-peptide levels, the duration of drug withdrawal, and serum C-peptide levels to ensure accurate assessment.