Abstract Background Due to high risk of mortality, children with comorbidities are typically excluded from trials evaluating pneumonia treatment. Understanding heterogeneity of outcomes among children with pneumonia and comorbidities is critical to ensuring appropriate treatment. Methods We explored whether the percentage of children with fast-breathing pneumonia cured at Day 14 was lower among those with selected comorbidities enrolled in a prospective observational study than among those enrolled in a concurrent randomized controlled trial evaluating treatment with amoxicillin in Lilongwe, Malawi. Results Among 79 children with fast-breathing pneumonia in the prospective observational cohort, 57 (72.2%) had HIV infection/exposure, 20 (25.3%) had malaria, 2 (2.5%) had severe acute malnutrition, and 17 (21.5%) had anemia. Treatment failure rate was slightly (not significantly) lower in children with comorbidities (4.1%, 3/73) compared to those without comorbidities (4.5%, 25/552) similarly treated. There was no significant difference in clinical cure rates by Day 14 (95.8% with vs 96.7% without comorbidity). Conclusions Children with fast-breathing pneumonia excluded from a concurrent clinical trial due to comorbidities did not fare worse. Children at higher risk whose caregivers seek care early and who receive appropriate risk assessment (e.g., pulse oximetry, hemoglobin, HIV/malaria testing) and treatment, can achieve clinical cure by Day 14. Trial registration ClinicalTrials.gov NCT02960919 ; registered November 8, 2016.