Current clinical practice guidelines recommend routine kidney function and serum potassium testing within 30 days of initiating angiotensin-converting enzyme inhibitor (ACEI) or angiotensin II receptor blocker (ARB) therapy. However, evidence is lacking on whether routine follow-up testing reduces therapy-related adverse events in adults with heart failure and if multimorbidity influences the association between laboratory testing and these adverse events. We conducted a retrospective cohort study among adults with heart failure from 4 US integrated health care delivery systems. Multimorbidity was defined using counts of chronic conditions. Patients with outpatient serum creatinine and potassium tests in the 30 days after starting ACEI or ARB therapy were matched 1:1 to patients without follow-up tests. We evaluated the association of follow-up testing with 30-day all-cause mortality and hospitalization with acute kidney injury or hyperkalemia using Cox regression. We identified 3629 matched adults with heart failure initiating ACEI or ARB therapy between January 1, 2005, and December 31, 2012. Follow-up testing was not significantly associated with 30-day all-cause mortality (adjusted hazard ratio [aHR] 0.45, 95% confidence interval [CI] 0.14; 1.39) and hospitalization with hyperkalemia (aHR 0.73, 95% CI, 0.33; 1.61). However, follow-up testing was significantly associated with hospitalization with acute kidney injury (aHR, 1.40, 95% CI, 1.01; 1.94). Interaction between multimorbidity burden and follow-up testing was not statistically significant in any of the outcome models examined. Routine laboratory monitoring after ACEI or ARB therapy initiation was not associated with risk of 30-day all-cause mortality or hospitalization with hyperkalemia across the spectrum of multimorbidity burden in a cohort of patients with heart failure.