Under-perception of asthma symptoms is associated with poor asthma outcomes. We assessed the effects of a behavioral intervention for improving perception of airflow limitation and asthma outcomes. A two-arm randomized controlled trial compared peak expiratory flow (PEF) feedback versus supportive counseling. Latino and Black adolescents with asthma ages 10-17 years old and caregivers were recruited from hospitals in the Bronx, NY. PEF feedback sessions reviewed accuracy of PEF guesses and medication adherence data, and targeted behavior change using motivational interviewing and problem-solving skills training. Supportive counseling received emotional support related to asthma. Both groups received 3 sessions across 6 weeks. All participants were blinded to PEF while guessing PEF during pre-intervention, 1, 6, and 12-month follow-up. Children in PEF feedback saw actual PEF after guesses were locked in during the 6-week intervention. Participants and assessors were blinded to group assignment. The primary outcome was under-perception of airflow limitation (divergence between actual PEF and guesses) on home spirometers. Secondary outcomes included daily PEF and forced expiratory volume in 1 second (FEV1), inhaled corticosteroid adherence measured by electronic monitors, Asthma Control Test, and emergency healthcare use for asthma. The sample comprised 354 children (M = 13.2±2.2 years; 62% Latino, 38% Black) and caregivers. PEF feedback (N = 153 analyzed) demonstrated greater improvements at 1-month follow-up on under-perception of airflow limitation (difference-in-differences, -12.64; 95% CI, -17.54 to -7.74), % personal best PEF (9.89; 95% CI, 7.13 to 12.65), % predicted FEV1 (4.93; 95% CI, 0.95 to 8.90) and ICS adherence (16.02; 95% CI, 7.15 to 24.89) compared with supportive counseling (N = 152 analyzed). At 12-month follow-up PEF feedback maintained improvements on under-perception of airflow limitation (-13.87; 95% CI, -19.03 to -8.71), higher PEF (14.23; 95% CI, 11.37 to 17.08) and %FEV1 (5.62; 95% CI, 1.56 to 9.67), and had smaller declines in ICS adherence (17.51; 95% CI, 7.12 to 27.89) versus pre-intervention than supportive counseling. No between-group differences existed for asthma control or healthcare use. The efficacy and sustainability of PEF feedback was established in improving children's perception of airflow limitation, pulmonary function, and medication adherence. Clinical trial registration available at www. gov, ID: NCT02702687.