Background: Regulatory T lymphocytes (Tregs) play an important role in acute rejection after lung transplantation. However, the importance of the time elapsed after transplantation on the Treg response requires further investigation.We aim to evaluate the change over time in the frequency of Tregs in lung allograft biopsies and to assess how Tregs relate to simultaneous and subsequent acute cellular rejection. Materials and methods: A total of 258 biopsy samples obtained 0.5, 1, 3, 12 and 24. months after transplantation from 58 consecutive lung transplant patients were included. The biopsies were scored for acute rejection according to the ISHLT criteria (A0-A4) and immunohistochemically stained with antibodies against FoxP3. Results: There was a tendency for a decrease in the number of Tregs/mm2 with time. However, the previous levels of Tregs/mm2 did not have any significant effect on future levels of Tregs/mm2. For biopsies taken 0.5 and 1month after transplantation, a significant correlation between Tregs/mm2 and the degree of acute rejection was found, and logistic regression analysis using updated values for Tregs/mm2 showed a significant relationship between Tregs/mm2 at 2weeks and an A-score ≥2 after 1 and 3months. At later time points, this correlation disappeared. Discussion and conclusion: Our data indicate that the time elapsed after transplantation is an important parameter influencing the Treg response after lung transplantation. This observation is in accordance with studies indicating a narrow therapeutic window for induction of tolerance by specifically targeting T-cells. The results also indirectly indicate that Tregs early after transplantation could have an impact on the long-term outcome.