Background: Biofilm infection of breast implants significantly potentiates capsular contracture. This study investigated whether chronic biofilm infection could promote T-cell hyperplasia. Methods: In the pig study, 12 textured and 12 smooth implants were inserted into three adult pigs. Implants were left in situ for a mean period of 8.75 months. In the human study, 57 capsules from patients with Baker grade IV contracture were collected prospectively over a 4-year period. Biofilm and surrounding lymphocytes were analyzed using culture, nucleic acid, and visualization techniques. Results: In the pig study, all samples were positive for bacterial biofilm. There was a significant correlation between the bacterial numbers and grade of capsular contracture (p = 0.04). Quantitative real-time polymerase chain reaction showed that all lymphocytes were significantly more numerous on textured compared with smooth implants (p < 0.001). T cells accounted for the majority of the lymphocytic infiltrate. Imaging confirmed the presence of activated lymphocytes. In the human study, all capsules were positive for biofilm. Analysis of lymphocyte numbers showed a T-cell predominance (p < 0.001). There was a significant linear correlation between the number of T and B cells and the number of detected bacteria (p < 0.001). Subset analysis showed a significantly higher number of bacteria for polyurethane implants (p < 0.005). Conclusions: Chronic biofilm infection around breast prostheses produces an increased T-cell response both in the pig and in humans. A possible link between bacterial biofilm and T-cell hyperplasia is significant in light of breast implant-associated anaplastic large-cell lymphoma.