Light from PbWO4 crystals in the barrel part of the CMS electromagnetic calorimeter will be detected by Avalanche Photodiodes (APDs). The crystal-APD system has to operate reliably in a high radiation environment. In the present paper, APD signal variation with wavelength and gain fluctuations for PbW04 scintillation light was investigated as a function of the neutron fluence. Calculations were made with a single-particle Monte Carlo simulation technique. The results show that the neutron fluence has an influence on the APD signal during 10 years CMS operation. (c) 2006 Elsevier B.V. All rights reserved.