In BNCT, local boron concentration, thermal neutron distribution and patient positioning largely determine this dose. In vivo measurements of these factors would considerably improve quality control and safety. Hydrogen gamma-rays emitted by the patient during therapy can serve as a measure of the thermal neutron distribution over the irradiated volume. They can be measured with spatial resolution by means of a scanning gamma-ray telescope. Measurements were compared with calculations with the treatment planning program (rtt_MC) and with MCNP. Measurements on several phantoms gave excellent agreement with calculations of the thermal neutron distribution. For patients, however, differences up to 20%2were observed. The patient measurements were reproducible between irradiation fractions.