Fractionation of mechanical wood pulp into two or more streams on the basis of fibre length has been investigated to recover valuable long fibres from virgin or recycled wood pulp. An air-sparged hydrocyclone was used to fractionate the pulp at feed mass concentrations (consistencies) of 0.15% and 0.30%. The extent of fibre fractionation, as affected by feed flow rate, air flow rate, ratio of overflow to underflow rates, was studied. As well, the effects of pore size of the air sparger and cyclone length are presented. It was found that at low consistencies of the pulp feed, the overflow stream from the hydrocyclone had a significantly longer average fibre length than that of the feed pulp. The feed pulp consistency, feed flow rate, air flow rate and the ratio of overflow to underflow flow rates had a direct effect on the fractionation performance.