Introduction Asthma control often is poor in adolescents and this causes considerable morbidity. Internet‐based self‐management (IBSM) improves asthma‐related quality of life in adults. We hypothesized that IBSM improves asthma‐related quality of life in adolescents. Methods Adolescents (12–18 years) with persistent and not well‐controlled asthma participated in a randomized controlled trial with 1 year follow‐up and were allocated to IBSM (n = 46) or usual care (UC, n = 44). IBSM consisted of weekly asthma control monitoring with treatment advice by a web‐based algorithm. Outcomes included asthma‐related quality of life (Pediatric Asthma Quality of Life Questionnaire, PAQLQ) and asthma control (Asthma Control Questionnaire, ACQ) and were analyzed by a linear mixed‐effects model. Results At 3 months, PAQLQ improved with 0.40 points (95% CI: 0.17–0.62, P < 0.01), by IBSM compared to 0.0 points for UC (P = 0.02 for the difference). At 12 months the between‐group difference was −0.05 (95% CI: −0.50 to 0.41, P = 0.85). At 3 months ACQ improved more in IBSM than in UC (difference: −0.32 points; 95% CI: −0.56 to −0.079, P < 0.01). At 12 months the difference was −0.05 (95% CI: −0.35 to 0.25, P = 0.75). Conclusion IBSM improved asthma‐related quality of life and asthma control in adolescents with not well‐controlled asthma after 3 months, but not after 12 months. Pediatr Pulmonol. 2012; 47:1170–1179. © 2012 Wiley Periodicals, Inc.