Abstract Clinical studies have shown that patients with non-small cell cancer (NSCLC) may be long-term survivors after irradiation, suggesting that radiotherapy may cure some of these tumors. Obviously, this represents an area where there is a need for greater understanding of the biology of the disease and for the development of suitable models to facilitate this. This report describes and compares the growth and radiosensitivity of a human lung adenocarcinoma cell line NC1-H125 in monolayer, as spheroids, and as xenografted tumors in nude mice. Multicellular spheroids are an in vitro tumor model system which is currently used in many aspects of cancer research. Monolayer cells were more radiosensitive than spheroids and had a lower capacity to repair sublethal damage as expressed by survival curve parameters, indicating a cell contact phenomenon. Comparing the growth delay and 50% cure doses in spheroids and xenografts, it was found that xenografts were more radioresistant than spheroids by a factor of 9. The intrinsic radiosensitivity of the tumor cells was not different in the two model systems. Since the degree of differentiation was approximately the same in both spheroids and xenografts, the differentiation grade is not likely to be a major determinant of radiosensitivity in these cells. The low growth rate of the xenografts indicates low vascularity and insufficient oxygen supply. Thus, the hypoxic cells in xenografts, which require a higher radiation dose to be sterilized, are responsible for the difference in radiation response of spheroids and xenografts. Hypoxia is a phenomenon that can be modelled in spheroids. By analyzing the radiation response concurrently in three model systems, we have shown that human tumor multicellular spheroids can model the radiation response of tumors in vivo. To the best of our knowledge, this is the first report on human non-small cell lung carcinoma grown as spheroids.