Purpose: Strength training activities have consistently been shown to improve running economy (RE) and neuromuscular characteristics, such as force producing ability and maximal speed, in adult distance runners. However the effects on adolescent (<18 years) runners remains elusive. This randomized control trial aimed to examine the effect of strength training on several important physiological and neuromuscular qualities associated with distance running performance. Methods: Participants (n=25, 13 female, 17.2 ±1.2 years) were paired according to their sex and RE and randomly assigned to a ten week strength training group (STG), or a control group (CG) who continued their regular training. The STG performed twice weekly sessions of plyometric, sprint and resistance training in addition to their normal running. Outcome measures included body mass, maximal oxygen uptake ( O2max), speed at O2max, running economy (quantified as energy cost), speed at fixed blood lactate concentrations (sFBLC), 20 m sprint, and maximum voluntary contraction (MVC) during an isometric quarter-squat. Results: Eighteen participants (STG, n=9, 16.1 ±1.1 years; CG, n=9, 17.6 ±1.2 years) completed the study. The STG displayed small improvements (3.2-3.7%, ES: 0.31-0.51) in running economy that were inferred as ‘possibly beneficial’ for an average of three submaximal speeds. Trivial or small changes were observed for body composition variables, O2max and s O2max, however the training period provided likely benefits to sFBLC in both groups. Strength training elicited a very likely benefit and a possible benefit to sprint time (ES: 0.32) and MVC (ES: 0.86) respectively. Conclusion: Ten weeks of strength training added to the programme of a postpubertal distance runner was highly likely to improve maximal speed, and enhances running economy by a small extent, without deleterious effects on body composition or other aerobic parameters.