BACKGROUND: Overuse of laboratory testing contributes substantially to health care waste, downstream resource use, and patient harm. Understanding patterns of variation in hospital-level testing across common inpatient diagnoses could identify outliers and inform waste-eduction efforts. METHODS: We conducted a multicenter retrospective cohort study of pediatric inpatients at 41 children's hospitals using administrative data from 2010 to 2016. Initial electrolyte testing was defined as testing occurring within the first 2 days of an encounter, and repeat testing was defined as subsequent testing within an encounter in which initial testing occurred. To examine if testing rates correlated across diagnoses at the hospital level, we compared risk-adjusted rates for gastroenteritis with a weighted average of risk-adjusted rates in other diagnosis cohorts. For each diagnosis, linear regression was performed to compare initial and subsequent testing. RESULTS: In 497 719 patient encounters, wide variation was observed across hospitals in adjusted, initial, and repeat testing rates. Hospital-specific rates of testing in gastroenteritis were moderately to strongly correlated with the weighted average of testing in other conditions (initial: r = 0.63; repeat r = 0.83). Within diagnoses, higher hospital-level initial testing rates were associated with significantly increased rates of subsequent testing for all diagnoses except gastroenteritis. CONCLUSIONS: Among children's hospitals, rates of initial and repeat electrolyte testing vary widely across 8 common inpatient diagnoses. For most diagnoses, hospital-level rates of initial testing were associated with rates of subsequent testing. Consistent rates of testing across multiple diagnoses suggest that hospital-level factors, such as institutional culture, may influence decisions for electrolyte testing.