Title: Poor adherence and limited treatment support are risk factors associated with second line treatment failure among children under 15 years at Victoria Chitepo provincial hospital, 2022: a case control study. Background: New HIV resistance mutations have been emerging and frequently changing. HIV positive people switching to third line have been increasing in Manicaland Province. Third line anti-retroviral therapy (ART) combinations are not always available. We sort to determine the factors associated with second line treatment failure among HIV positive patients. Methods: We conducted a 1:1 unmatched case-control study among 107 case-control pairs. A case was any HIV positive individual with treatment failure confirmed by 2 viral load of ≥1000 copies/ml while a control was any HIV positive individual with a viral load of <1000/ml copies; on second line therapy at Victoria Chitepo Provincial Hospital (VCPH), in 2022. Systematic random sampling was used to select cases and controls from the line-list. Administered questionnaires were used to ascertain risk factors to second line treatment failure. Patient booklets were reviewed for demographic and clinical characteristics. Frequencies, means, medians and odds ratios were generated. Multivariate analysis and stratified analysis was performed to identify independent risk factors. Permission to proceed was sought from relevant authorities and all ethical considerations were observed. Results: Defaulting treatment (aOR=2.42, 95% CI: 1.03-5.93), missing a viral load test (aOR=5.52, 95% CI: 2.30-13.26) and being under 15 years of age (aOR=6.18, 95% CI: 1.46-26.20) were independent risk factors associated with second line treatment failure. Having a treatment support was an independent protective factor (aOR=0.30, 95% CI: 0.10-0.86). Conclusion: Poor adherence is the key drive to second line treatment failure. Good adherence is critical in children under 15 years. Treatment support promotes viral load suppression.