Lung function testing has been suggested to provide a potential risk regarding cross-infection between patients. About 155 patients (86 infectious, 69 non-infectious) used a single use bacterial/viral filter when performing routine lung function tests. Swabs from the patient side of the filter (Proximal) and the equipment side (Distal), and two sections of the filter itself were cultured. About 33/155 samples showed bacterial growth on the Proximal compared with 2/155 on the Distal side (P<0.01). Growth was obtained from the filter in 125/155 (80.6%) of cases. Pathogenic micro-organisms such as Pseudomonas aeruginosa (4 cases) and Staphylococcus aureus (5 cases) were isolated. Appropriate infection control measures should be used when performing lung function tests.