BACKGROUND AND OBJECTIVES: International patterns of antibiotic use and laboratory testing in bronchiolitis in emergency departments are unknown. Our objective is to evaluate variation in the use of antibiotics and nonindicated tests in infants with bronchiolitis in 38 emergency departments in Pediatric Emergency Research Networks in Canada, the United States, Australia and New Zealand, the United Kingdom and Ireland, and Spain and Portugal. We hypothesized there would be significant variation, adjusted for patient characteristics. METHODS: We analyzed a retrospective cohort study of previously healthy infants aged 2 to 12 months with bronchiolitis. Variables examined included network, poor feeding, dehydration, nasal flaring, chest retractions, apnea, saturation, respiratory rate, fever, and suspected bacterial infection. Outcomes included systemic antibiotic administration and urine, blood, or viral testing or chest radiography (CXR). RESULTS: In total, 180 of 2359 (7.6%) infants received antibiotics, ranging from 3.5% in the United Kingdom and Ireland to 11.1% in the United States. CXR (adjusted odds ratio [aOR] 2.3; 95% confidence interval 1.6-3.2), apnea (aOR 2.2; 1.1-3.5), and fever (aOR 2.4; 1.7-3.4) were associated with antibiotic use, which did not vary across networks (P = .15). In total, 768 of 2359 infants (32.6%) had ≥1 nonindicated test, ranging from 12.7% in the United Kingdom and Ireland to 50% in Spain and Portugal. Compared to the United Kingdom and Ireland, the aOR (confidence interval) results for testing were Canada 5.75 (2.24-14.76), United States 4.14 (1.70-10.10), Australia and New Zealand 2.25 (0.86-5.74), and Spain and Portugal 3.96 (0.96-16.36). Testing varied across networks (P < .0001) and was associated with suspected bacterial infections (aOR 2.12; 1.30-2.39) and most respiratory distress parameters. Viral testing (591 of 768 [77%]) and CXR (507 of 768 [66%]) were obtained most frequently. CONCLUSIONS: The rate of antibiotic use in bronchiolitis was low across networks and was associated with CXR, fever, and apnea. Nonindicated testing was common outside of the United Kingdom and Ireland and varied across networks irrespective of patient characteristics.