The objective of this study was to determine the level of adherence of pediatricians in Spain to the Spanish National Guidelines for Asthma Treatment with regard to the use of a peak flow meter (PEFR) or a spirometer in the diagnosis and management of asthma in childhood and to analyze sources of variations in these practices. A prospective survey (consisting of demographic and asthma knowledge sections) was conducted over a 2-wk time interval of 3000 pediatricians throughout the country. At least one part of the questionnaire was completed and returned by 2773 individuals (92.4%), with 2347 (78.2%) answering both sections; results are for this population. Around 62% of the pediatricians reported having a peak flow meter or a spirometer in their office; however, only 33% and 48% of them used the devices for the diagnosis and treatment of asthma, respectively. There was a significant association between being older (36-55 yr old) and using PEFR or spirometry for the diagnosis (OR: 1.35, 95% CI 1.11-1.66) and the management (OR 1.47, 95% CI 1.22-1.77) of asthma. Males used a peak flow meter or a spirometer more often than females for the diagnosis (37.8% vs. 30.9%, p = 0.001) and management of asthma (52.0% vs. 45.6%, p = 0.008). Pediatricians with formal pediatric residence training used these devices more for the diagnosis (OR: 1.39, 95% CI 1.09-1.75) and management (OR: 1.58, 95% CI 1.27-1.96) than those without. Working in a hospital was also related with more peak flow meter or spirometer use than working in health centers (OR: 2.08, 95% CI 1.71-2.54 for diagnosis; OR: 1.83, 95% CI 1.50-2.22 for management). About one-third of the Spanish pediatricians surveyed use spirometers and/or peak flow meters for diagnosing asthma and about half use one of these devices occasionally for managing the disease. Independent factors favoring their use are: age 36-55 yr, male gender, working in a hospital setting, and having been trained in a formal pediatric residence program.