This study aimed to determine whether osteopathic manipulation of the T9-T10 vertebrae improves the evolution of tonsillitis. A randomized, stratified, controlled clinical trial with blinded patients, evaluator and data analyst was performed. The patients in the control group (CG) underwent a "sham" manipulation. A high-speed, low-amplitude technique was applied to the T9-T10 vertebrae in the osteopathic manipulative group (OMG) patients. The number of days needed to resolve the tonsillitis was significantly lower (p = 0.025) in the OMG (2.03 ± 0.95 days) than the CG (2.39 ± 0.82 days). Additionally, the number of episodes of tonsillitis after the treatment decreased significantly more in the OMG (0.8 ± 1.88 episodes/year in total) than the CG (2 ± 2.12) (p = 0.005). In the OMG, 60.8% had no recurrences of tonsillitis, compared to 22.5% of the CG, in the following year (χ2 (1) = 15.57, p < 0.001). No patients reported adverse effects. It has been concluded that during an episode of tonsillitis, the number of days to resolution was significantly lower after the application of an osteopathic manipulation of the T9-T10 vertebrae, compared to a sham manipulation. The number of subsequent year tonsillitis episodes was greatly reduced in both groups, significantly more in the OMG than in the CG patients.