After a brief review of existing knowledge concerning the effect of ambient pressure on spray drop size, results are presented for a swirl nozzle at fuel Δp’s of 25 and 100 psi for ambient gas pressures of 0.5, 14.5, and 114.5 psia. The liquid sprayed is diesel fuel, the nozzle capacity being 45 gal/hr at 100 psi Δp, and the nominal spray angle, 80 deg. The photographic method by which drop size was determined is described. Curves are presented which show the spatial variation in fuel flow rate, spray-stream velocity, and drop size. The notable effects are large drop size and velocity variations across the spray stream, and for the total nozzle output, an increase in the drop size as the ambient pressure goes from 14.5 to 114.5 psia. Some ramifications of the results are discussed.