There is continuing concern about whether and to what extent airliner cabins represent an increased risk of transmission of airborne infectious disease. The purpose of this study was to examine through a simple experiment the relative importance of close proximity and partial recirculation of cabin air on the potential risk of disease transmission. Results are presented from measurements of instantaneous point source dispersion in a cabin on a commercial airline flight. A small amount of tracer gas was released as a puff in the passenger cabin of a wide body jet at cruise altitude. Tracer gas samples were taken manually in the period immediately after the release by two technicians sitting 2 m and 30 m forward of the release point in the cabin. The maximum tracer concentration observed at the 2 m sampling point occurred at 5 s after the release and was a factor of 500 greater than the maximum observed at the 30 m sampling point, which occurred 6.5 min after the release. The integrated tracer exposure at the 2 m location was approximately a factor of 30 greater than at the 30 m location. Assuming risk of transmission is proportional to dose, then the results support the hypotheses that infectious diseases are transmitted primarily between people sitting in close proximity to each other in an aircraft cabin and that partial recirculation of ventilation air in the cabin has a negligible impact on occupants' risk of exposure.