Although EMLA is known to be an effective topical anesthetic, its rate of success is unknown. Indeed, researchers have suggested that EMLA may fail with young and apprehensive children. Therefore, the objectives of this study were to assess EMLA's rate of success as well as factors which predict success. A double-blind, placebo-controlled design was utilized. The sample included 258 children and adolescents aged 5-18 years who were having venipuncture or intravenous (i.v.) cannulation. After having their anxiety assessed, subjects were randomly assigned to have EMLA or placebo applied over the procedure site for 90 min. The visual analogue scale was used to assess pain caused by removal of the semi-permeable dressing and by the procedure. Other information that was collected included: duration of drug application, interval between drug removal and procedure, skin changes at bandage and drug sites and rated difficulty of the procedure. EMLA was successful 84% of the time for venipuncture and 51% of the time for i.v. cannulation. Factors which predicted success of EMLA included type of procedure, duration of drug application and anxiety. EMLA was less successful for i.v. cannulation compared to venipuncture even with duration of drug application controlled. Those who had a poor outcome were more anxious than those with a good outcome. Age of child was not a factor. Strategies for improving efficient use of EMLA were recommended.