BACKGROUND Residual neuromuscular blockade (RNMB) is known to be associated with respiratory complications in the postoperative period after muscle relaxant usage. The authors hypothesized that RNMB causes reductions in pulmonary function test (PFT) parameters in the immediate postoperative period. METHODS An open-label prospective randomized cohort study was conducted comparing reductions in PFT parameters due to RNMB among different neuromuscular blocking agents. One hundred and fifty patients were randomized to receive vecuronium, atracurium, or rocuronium. After reversal of neuromuscular blockade and extubation, train-of-four ratio was measured every 5 min until the train-of-four ratio of 0.9 or greater was attained. PFTs were performed preoperatively and postoperatively when the patients were willing and fit. The train-of-four ratio, measured at PFT, was used to classify patients into "RNMB absent" and "RNMB present." RNMB was defined as a train-of-four ratio less than 0.9. RESULTS Thirty-nine patients had RNMB at the time of performing PFT. There was no statistically significant difference in the postoperative reductions in PFT parameters in patients with RNMB among different neuromuscular blocking agents. Patients were regrouped as RNMB absent and RNMB present, irrespective of neuromuscular blocking agents. Postoperative PFT values for the RNMB-absent and RNMB-present groups were 62% and 49% of baseline forced vital capacity and 47% and 38% of baseline peak expiratory flow of the baseline, respectively. Postoperative forced vital capacity and peak expiratory flow values of RNMB-present patients were lower by 13% and 9% in absolute terms (P<0.008) and 21% and 19% in relative terms, respectively, compared with RNMB-absent patients. CONCLUSIONS RNMB results in reductions in forced vital capacity and peak expiratory flow in the immediate postoperative period indicating impaired respiratory muscle function.