The two-year long study in the Journal of the American Medical Association (JAMA) included nearly 500 overweight volunteers who were asked to diet and take more exercise.
Half were given a fitness tracker to help them keep tabs.
This group had lost less weight than the other one by the end of the trial.
The study authors say this does not mean people should ditch the technology altogether, but neither should they put too much faith in them, at least as a slimming aid.
