Carrying loads close to the trunk with a backpack causes a restrictive type of change in lung function in which Forced Vital Capacity (FVC) and Forced Expiratory Volume in 1 s (FEV1) are reduced without a corresponding decrement in the FEV1.FVC( - 1) %. It is not known whether this is due to the weight of the load acting on the chest or to the tightness of fit of the shoulder and chest straps and waist belt of the pack harness. This study examined FVC, FEV1, FEV1.FVC( - 1) %, peak expiratory flow (PEF), forced expiratory flow between 0.2 and 1.2 s (FEF0.2 - 1.2) after the start of expiration and between 25 and 75% of each FVC (FEF25 - 75%) in 12 healthy males wearing a 15 kg backpack in which the shoulder and chest straps and hip belt were loosened by 3 cm from a 'comfort fit' to achieve a 'loose pack' fit (LPF) and tightened by 3 cm from CF to achieve a 'tight pack' fit (TPF). In comparison with the control condition of no pack, a loose pack fit significantly reduced FVC (by 3.6%, p < 0.01), FEV1 (by 4.3%, p < 0.01) and FEF25 - 75% (by 8.4%, p < 0.01). A tight pack fit significantly reduced FVC (by 8.1%, p < 0.01) and FEV1 (by 9.1%, p < 0.001). It also significantly reduced FEF0.2 - 1.2 (by 7.3%, p < 0.05) and FEF25 - 75% (by 21%, p < 0.01). In comparison with a loose pack fit, the tight pack fit was associated with a significantly lower FVC (by 4.6%, p < 0.01), FEV1 (by 5.0%, p < 0.01), FEF25 - 75% (by 13.8%, p < 0.01) and a fall in FEF0.2 - 1.2 (by 5.5%). The latter was approaching significance (p = 0.077). There were no significant changes in FEV1.FVC( - 1)% and PEF. It is concluded that tightening the fit of a backpack significantly affects lung function in a manner that is typical of a restrictive change in lung function and is very similar in pattern to that of wearing a loosely fitted loaded backpack. The effect of tightness of fit is additional to that due to the weight of the load alone and may also reduce expiratory flow at low lung volumes.