The development and the preliminary evaluation of a mathematical model used to design a horizontal-flow anaerobic immobilized biomass (HAIB) reactor applied to domestic sewage treatment is presented. The influences of both the liquid superficial velocity and the bioparticle size on the overall reaction rate were examined. The size of the particle used as biomass support was found to affect strongly the total volume of the designed unit since the effectiveness factor decreased as the bioparticle size increased. On the contrary, the overall organic matter conversion rate was not substantially influenced by the liquid velocity. Based on these data obtained from the mathematical model, a pilot-scale HAIB reactor was constructed adopting some design parameters and following the design criteria established. The HAIB reactor comprised a 14.5 cm internal diameter PVC tube with length of 14.4 m divided in to five stages of 2.88 m each. Polyurethane foam cubic matrices with 1 cm side were used as support for biomass immobilization. The reactor was operated at a mean hydraulic retention time of 4.3 h at mean sewage temperature of 25 +/- 1 degrees C and mean liquid flow rate of 20 L h(-1). A preliminary evaluation of the mathematical model proposed to design the HAIB reactor was performed based on data of chemical oxygen demand (COD) profiles along the reactor length. Although deviations were observed between experimental and theoretical data, the proposed model was found to be suitable for predicting the overall behavior of the reactor.