We hypothesized that peak values of oesophageal (Poes) and transdiaphragmatic pressure (Pdi) swings during a maximal sniff manoeuvre and a maximal static inspiratory manoeuvre (Muller manoeuvre) are comparable or give complementary information for assessing diaphragmatic and global inspiratory muscle strength. We studied 98 patients with suspected diaphragmatic dysfunction. Poes and Pdi swings were measured during maximal sniff manoeuvres (sniff), maximal Muller manoeuvres (max), and cervical magnetic phrenic nerve stimulation (cervical Tw). Eighty eight patients were able to perform both volitional manoeuvres. Among them, mean Poes sniff was significantly higher than mean Poes max (48.7+/-28.7 cm H(2)O vs. 42.9+/-27.4 cm H(2)O, p<0.05) and mean Pdi sniff was higher than mean Pdi max (49.2+/-35.1cm H(2)O vs. 42.9+/-33.3 cm H(2)O, respectively, p=0.05). Cervical Pdi Tw correlated better with Pdi sniff (p<0.0001, r=0.62) than with Pdi max (p<0.0001, r=0.44). Poes and Pdi swings were greatest during the sniff manoeuvre in 42 patients (48%) and during the Muller manoeuvre in 29 patients (33%). Among the 17 remaining patients, nine had the greatest Poes swing during a maximal sniff manoeuvre and the greatest Pdi swing during a maximal static inspiratory manoeuvre; the opposite occurred in the other eight patients. The combination of Muller manoeuvre and sniff manoeuvre increased the diagnosis of normal diaphragmatic strength from 18 patients (20%) to 21 patients (24%), and the additional analysis of cervical Pdi Tw further increased the diagnosis of normal diaphragmatic strength to 27 patients (31%). In conclusion, though sniff manoeuvre gave significantly higher values than Muller manoeuvre, both volitional manoeuvres and cervical Pdi Tw are complementary and should be used in combination to evaluate diaphragmatic muscle strength.