BACKGROUND Accurate assessment of diaphragm strength can be difficult. Transdiaphragmatic pressure (PDI) measurements during volitional manoeuvres are useful but it may be difficult to ensure maximum patient effort. Magnetic stimulation of the phrenic nerves is easy to perform and the results are reproducible in normal subjects. The purpose of the present study was to evaluate the usefulness of magnetic stimulation of the phrenic nerves in the assessment of diaphragm weakness in patients. METHODS Sixty-six patients referred for assessment of respiratory muscle strength and 23 normal subjects were studied. Twitch PDI (TwPDI) following magnetic stimulation of the phrenic nerves and sniffPDI were obtained in all individuals. TWPDI following bilateral electrical stimulation of the phrenic nerves was also obtained in eight patients. RESULTS Mean (SD) TwPdi for the normal subjects was 31 (6) cm H2O and 18 (11) cm H2O for the patients. TwPDI and sniffPDI were correlated (r = 0.77). Seven of the 37 patients (19%) with a reduced sniffPDI had a TwPDI within the normal range whereas two of the 32 patients (6%) with a reduced TwPDI had a normal sniffPDI. TwPDI was similar with magnetic and electrical stimulation. CONCLUSIONS TwPDI following magnetic stimulation of the phrenic nerves is a clinically useful measurement when assessing diaphragm weakness.