Purpose <p>This study documented changes in cartilage lesions in the arthritic medial knee compartment after medial opening wedge high tibial osteotomy (MOWHTO), their impact on clinical outcomes and possible factors influencing this improvement.</p> Methods <p>Forty-nine patients indicated for MOWHTO (per ISAKOS criteria) underwent arthroscopy at osteotomy and implant removal (mean interval: 22.2 months). Cartilage lesions (medial femoral condyle [MFC], medial tibial plateau [MTP]) and clinical scores (Lysholm score and KOOS) were documented. No cartilage restoration procedures were performed.</p> Results <p>The mean age was 47.5 years and most were females (<i>n</i> = 32). Arthroscopically, cartilage improvement occurred in 32 (65.3%) MFC and 25 (51%) MTP lesions. MOWHTO significantly improved the mechanical tibiofemoral angle (TFA) (7.88 ± 3.66 vs. − 2.71 ± 1.98 (°); <i>P</i> &lt; 0.001). Lysholm knee score (77 ± 16.2 vs. 48.4 ± 16.3, <i>p</i> &lt; 0.001) and KOOS (64.1 ± 13.6 vs. 43.8 ± 12.5, <i>p</i> &lt; 0.001) significantly improved after osteotomy. On the femoral side, cartilage repair was documented in patients who achieved relatively more valgus correction, with a mean final TFA of 3.32 ± 1.7, compared with 1.57 ± 2 in the no improvement group (<i>P</i> = 0.001). For the MTP, no factor of statistical significance could be detected.</p> Conclusions <p>MOWHTO can improve articular cartilage lesions as a standalone procedure. A greater incidence of cartilage repair is expected with more valgus correction of the mechanical axis of the limb. This cartilage infill is associated with slightly better clinical outcomes.</p> Trial registration <p>ClinicalTrials.gov (ID NCT04541342) registered on 9.9.2020, <a href="https://wclinicaltrials.gov/study/NCT04541342">https://clinicaltrials.gov/study/NCT04541342</a>.</p>