A method and system for monitoring a gas turbine engine (20) to predict maintenance requirements. Particles suspended in a gas flow (24, 32) of the engine (20) are monitored and quantified to predict a particle accumulation rate. Monitoring may be done using particle flow sensors (61-63) in a diverted portion (33) of the working gas flow (24), such as in the cooling gas flow (32). Particle sampling (S1-S3) may be done to determine particle size and composition distributions. Particle mass flow rates may then be continuously monitored per engine operating condition, and compared to predetermined values such as a normal upper limit per engine operating condition. An integrated particle mass flow may be used in conjunction with an instantaneous mass flow rate to predict a maintenance requirement. Multiple locations (L1-L3) may be monitored to recognize a maintenance requirement by flow section or component.