The first direct in situ measurements of HO₂NO₂ in the upper troposphere were performed from the NASA DC-8 during the Intercontinental Chemical Transport Experiment–North America 2004 with a chemical ionization mass spectrometer (CIMS). These measurements provide an independent diagnostic of HO_x chemistry in the free troposphere and complement direct observations of HO_x, because of the dual dependency of HO₂NO₂ on HO_x and NO_x. On average, the highest HO₂NO₂ mixing ratio of 76 pptv (median = 77 pptv,σ = 39 pptv) was observed at altitudes of 8–9 km. Simple steady state calculations of HO₂NO₂, constrained by measurements of HO_x, NO_x, and J values, are in good agreement (slope = 0.90, R² = 0.60, and z = 5.5–7.5 km) with measurements in the midtroposphere where thermal decomposition is the major loss process. Above 8 km the calculated steady state HO₂NO₂ is in poor agreement with observed values (R² = 0.20) and is typically larger by a factor of 2.4. Conversely, steady state calculations using model-derived HO_x show reasonable agreement with the observed HO₂NO₂ in both the midtroposphere (slope = 0.96, intercept = 7.0, and R² = 0.63) and upper troposphere (slope = 0.80, intercept = 32.2, and R² = 0.58). These results indicate that observed HO₂ and HO₂NO₂ are in poor agreement in the upper troposphere but that HO₂NO₂ levels are consistent with current photochemical theory.