Biomass growth models for 13-year-old maritime pine tree stands (Pinus pinaster Ait.) in the north-eastern Portugal were developed and used to analyse the effects of the defoliation by the pine processionary moth, Thaumetopoea pityocampa (Den. & Schiff.) on biomass increment. For the adjustment of the models, 30 individual pine trees were destructively sampled and non-linear models were tested, using the diameter at 10 centimetre height (d0.10), the total height (h), both variables (d0.10+h) and d0.10 2h as preditors of biomass growth. The results showed that the best predictor was d0.10+h. Application of models to analyse tree biomass after attack by the pine processionary moth showed that the decrease of biomass increment was proportional to the severity of the insect attack, with average values of losses in biomass increment ranging from 37% to 73%, depending on defoliation intensity.