To assess whether the accepted weaning parameters (derived from middle-aged patients) are helpful in determining discontinuation of mechanical ventilatory support (MVS) in elderly patients, we retrospectively reviewed records of 269 patients greater than or equal to 70 yr who were weaned from MVS from January, 1984 through June, 1985 at one institution. Parameters studied included spontaneous respiratory rate, tidal volume, minute ventilation, maximal inspiratory pressure (MIP), pH, PaCO2, PaO2, and PaO2/FiO2. Two hundred forty-one patients (mean age 80 yr) were successfully weaned while 28 patients (mean age 80 yr) required reinstitution of MVS within 48 h. MIP and pH were statistically lower in the unsuccessfully weaned group, although the mean absolute differences were small (-32 vs. -38 cm H2O and 7.42 vs. 7.44 cm H2O, respectively). All parameters had good positive predictive values but poor negative predictive values (less than or equal to 22%) and only marginal diagnostic accuracy (58% to 86%). We conclude that strict adherence to previously published weaning parameters may not be applicable in deciding when to discontinue MVS in elderly patients.