Exhaust gas treatment system for an internal combustion engine, the system comprising: - a particulate open-filter, - a Diesel Oxidation Catalyst placed upstream of the open-filter, according to a direction of the exhaust gas flow, - active regeneration means for controlling the active regeneration of the open-filter as a function of an amount of particulate matter accumulated in the open-filter, wherein said active regeneration means comprise an estimator at least for estimating the amount of particulate matter accumulated in the open-filter; said estimator comprising: - first calculating means (B1,B2) for estimating a first amount/rate of particulate matter produced by the internal combustion engine as a function of operating conditions of the internal combustion engine, - second calculating means for estimating a second amount/rate of particulate matter regenerated by means of passive reactions in the open-filter; - adjusting means for weighing the estimated first amount/rate with a coefficient (η) indicative of a retaining efficiency of the open-filter so as to obtain a weighed third amount/rate of particulate matter; - third calculating means for subtracting the estimated second amount/rate from the weighed third amount/rate; characterised in that said coefficient (η) is a value that is variable in time as a function of actual operating conditions of the internal combustion engine, and said adjusting means comprise at least one relation to determine said variable value as a function of actual operating conditions of the internal combustion engine; wherein said actual operating conditions comprise: - variable input values of a first parameter, indicative of an actual rotation speed of said internal combustion engine, and - variable input values of a second parameter, indicative of an actual drive torque or an actual power of said internal combustion engine. - a particulate open-filter, - a Diesel Oxidation Catalyst placed upstream of the open-filter, according to a direction of the exhaust gas flow, - active regeneration means for controlling the active regeneration of the open-filter as a function of an amount of particulate matter accumulated in the open-filter, wherein said active regeneration means comprise an estimator at least for estimating the amount of particulate matter accumulated in the open-filter; said estimator comprising: - first calculating means (B1,B2) for estimating a first amount/rate of particulate matter produced by the internal combustion engine as a function of operating conditions of the internal combustion engine, - second calculating means for estimating a second amount/rate of particulate matter regenerated by means of passive reactions in the open-filter; - adjusting means for weighing the estimated first amount/rate with a coefficient (η) indicative of a retaining efficiency of the open-filter so as to obtain a weighed third amount/rate of particulate matter; - third calculating means for subtracting the estimated second amount/rate from the weighed third amount/rate; - first calculating means (B1,B2) for estimating a first amount/rate of particulate matter produced by the internal combustion engine as a function of operating conditions of the internal combustion engine, - second calculating means for estimating a second amount/rate of particulate matter regenerated by means of passive reactions in the open-filter; - adjusting means for weighing the estimated first amount/rate with a coefficient (η) indicative of a retaining efficiency of the open-filter so as to obtain a weighed third amount/rate of particulate matter; - third calculating means for subtracting the estimated second amount/rate from the weighed third amount/rate; characterised in that said coefficient (η) is a value that is variable in time as a function of actual operating conditions of the internal combustion engine, and said adjusting means comprise at least one relation to determine said variable value as a function of actual operating conditions of the internal combustion engine; wherein said actual operating conditions comprise: - variable input values of a first parameter, indicative of an actual rotation speed of said internal combustion engine, and - variable input values of a second parameter, indicative of an actual drive torque or an actual power of said internal combustion engine. - variable input values of a first parameter, indicative of an actual rotation speed of said internal combustion engine, and - variable input values of a second parameter, indicative of an actual drive torque or an actual power of said internal combustion engine.