The first calibrated in situ measurements of hydrogen cyanide (HCN) concentrations in the upper troposphere/lower stratosphere were made during the March 2000 deployment of the SAGE III-Ozone Loss and Validation Experiment (SOLVE) campaign. The measurements were made from the NASA DC-8 aircraft flying at cruise altitudes between 10 and 12.5 km in the northern polar region. The average HCN volume mixing ratio was found to be 280 ¿ 48 (1σ) pptv. No concentration gradient was observed between tropospheric and stratospheric air, and no HCN emissions from the Mount Hekla volcano were found. A very slight increase in mixing ratio with altitude was detected, although it is within the uncertainty in the data. The altitude dependence agrees with recent model calculations. The measurements agree with the most recent concentrations derived from total column densities but are higher than earlier measurements.