BACKGROUND AND OBJECTIVES: Poorly designed labels and dosing tools contribute to dosing errors. We examined the degree to which errors could be reduced with pictographic diagrams, milliliter-only units, and provision of tools more closely matched to prescribed volumes. METHODS: This study involved a randomized controlled experiment in 3 pediatric clinics. English- and Spanish-speaking parents (n = 491) of children ≤8 years old were randomly assigned to 1 of 4 groups and given labels and dosing tools that varied in label instruction format (text and pictogram, or text only) and units (milliliter-only ["mL"] or milliliter/teaspoon ["mL/tsp"]). Each parent measured 9 doses of liquid medication (3 amounts [2, 7.5, and 10 mL] and 3 tools [1 cup, 2 syringes (5- and 10-mL capacities)]) in random order. The primary outcome was dosing error (>20% deviation), and large error (>2× dose). RESULTS: We found that 83.5% of parents made ≥1 dosing error (overdosing was present in 12.1% of errors) and 29.3% of parents made ≥1 large error (>2× dose). The greatest impact on errors resulted from the provision of tools more closely matched to prescribed dose volumes. For the 2-mL dose, the fewest errors were seen with the 5-mL syringe (5- vs 10-mL syringe: adjusted odds ratio [aOR] = 0.3 [95% confidence interval: 0.2–0.4]; cup versus 10-mL syringe: aOR = 7.5 [5.7–10.0]). For the 7.5-mL dose, the fewest errors were with the 10-mL syringe, which did not necessitate measurement of multiple instrument-fulls (5- vs 10-mL syringe: aOR = 4.0 [3.0–5.4]; cup versus 10-mL syringe: aOR = 2.1 [1.5–2.9]). Milliliter/teaspoon was associated with more errors than milliliter-only (aOR = 1.3 [1.05–1.6]). Parents who received text only (versus text and pictogram) instructions or milliliter/teaspoon (versus milliliter-only) labels and tools made more large errors (aOR = 1.9 [1.1–3.3], aOR = 2.5 [1.4–4.6], respectively). CONCLUSIONS: Provision of dosing tools more closely matched to prescribed dose volumes is an especially promising strategy for reducing pediatric dosing errors.