Objective: Shortage of donor organs for children has led to the use of living related adult lung lobar transplants. It is not known how these lobes or the recipient remaining lung grow after such transplants. The purpose of the present study was to assess lung growth in rat lungs up to 6 months after adult lobe transplantation into a juvenile recipient. Methods: Right cardiac lung lobes from adult male Lewis rats were transplanted into the left hemithorax of juvenile (6-week-old) male Lewis rats after left pneumonectomy. Animals with appropriate controls were put to death 14 days and 6 months after transplantation. The lungs were fixed inflated and studied by means of quantitative morphometric techniques. Results: By 6 months after transplantation both the recipient right lung and the transplanted cardiac lobe were significantly larger than normal (p = 0.005; p = 0.001). In the recipient right lung this increase was due to an increase in the number of alveoli (p = 0.004) and in the transplanted cardiac lobe to an increase in size of the alveoli (p = 0.008). Conclusions: An adult lobe transplanted into a young recipient is still viable and has normal architecture after 6 months, and growth of the recipients' own lung continues. The outlook for comparable transplants in children is promising, although the human condition can be complicated by rejection, infection, and treatment strategies.