The influence of lung tissue on depth dose distributions, produced by beams of fast neutrons (generated at the Naval Research Laboratory cyclotron by bombarding a thick Be target with 35‐MeV deuterons) was determined. Muscle‐equivalent ionization chambers were utilized in conjunction with thermoluminescent dosimeters to establish the neutron and gamma ray‐dose deposited in a phantom of the thoracic region of the body containing lung components. All irradiations were carried out at a target–skin distance of 125 cm with neutron beams of 10 × 10‐cm field size at the surface of the phantom. From these measurements, the authors conclude that the presence of lung tissue results in higher dose levels for points at depths greater than the midpoint of the lungs as a result of the increased transmission of neutrons by low‐density lung tissue. An examination of the dose measurements revealed that the portion of the total dose due to gamma rays is of the order of 4–10% for points in lung and muscle‐equivalent fluid.