As part of a randomised controlled trial of treatment with placebo 3 days of amoxicillin for nonsevere fast-breathing pneumonia among Malawian children aged 2-59 months, a subset of children was hospitalised for observation. We sought to characterise the progression of fast-breathing pneumonia among children undergoing repeat assessments to better understand which children do and do not deteriorate. Vital signs and physical examination findings, including respiratory rate, arterial oxygen saturation measured by pulse oximetry ( ), chest indrawing and temperature were assessed every 3 h for the duration of hospitalisation. Children were assessed for treatment failure during study visits on days 1, 2, 3 and 4. Hospital monitoring data from 436 children were included. While no children had 90-93% at baseline, 7.4% (16 of 215) of children receiving amoxicillin and 9.5% (21 of 221) receiving placebo developed 90-93% during monitoring. Similarly, no children had chest indrawing at enrolment, but 6.6% (14 of 215) in the amoxicillin group and 7.2% (16 of 221) in the placebo group went on to develop chest indrawing during hospitalisation. Repeat monitoring of children with fast-breathing pneumonia identified vital and physical examination signs not present at baseline, including 90-93% and chest indrawing. This information may support providers and policymakers in developing guidance for care of children with nonsevere pneumonia.