The effect of fast neutron irradiation on the properties of wide-base p-i-n diodes has been investigated both theoretically and experimentally. Dependence of the base voltage on irradiation dose was studied for diodes at intermediate level injection. The change in resistivity of silicon under neutron irradiation should be taken into account as well as change in the carrier lifetime. The main contribution to the I-V curve shift of the wide-base p-i-n diode arises from the resistivity change for doses larger than 20-30 Gy. Such diodes have a high radiation sensitivity (above 5 V. Gy-1 for diodes with base width of 0.5 cm made of silicon with resistivity of 5 x 104 Ω · cm). Using p-i-n diodes at intermediate level injection one can significantly widen the measurement range of fast neutron doses.