At present positron emission tomography (PET) is the only feasible method of an in situ and non-invasive monitoring of patient irradiation with ions. At the experimental carbon ion treatment facility of the Gesellschaft für Schwerionenforschung (GSI) Darmstadt an in-beam PET scanner has been integrated into the treatment site and lead to a considerable quality improvement of the therapy. Since ions other than carbon are expected to come into operation in future patient treatment facilities, it is highly desirable to extend in-beam PET also to other therapeutic relevant ions, e.g. (7)Li. Therefore, by means of the in-beam PET scanner at GSI the beta(+)-activity induced by (7)Li(3+) ions has been investigated for the first time. Targets of PMMA, water, graphite and polyethylene were irradiated with monoenergetic, pencil-like beams of (7)Li(3+) with energies between 129.1 A MeV and 205.3 A MeV and intensities ranging from 3.0 x 10(7) to 1.9 x 10(8) ions s(-1). This paper presents the measured beta(+)-activity profiles as well as depth dependent thick target yields which have been deduced from the experimental data. The beta(+)-activity induced by (7)Li ions was found to be a factor of 1.76 higher than the one induced by (12)C ions at the same physical dose and particle range.