The brachial plexus was identified by electrical stimulation before interscalene block with 30 mL 0.5% bupivacaine and adrenaline 1:200 000. During injection, compresion was applied with a finger proximal to the injection site. Spirometric measurements were made before the block, and then at 5 min, 10 min, 20 min and 4 h after the injection. Diaphragmatic excursion was measured radiographically before the block, and at 15 min and 4 h afterwards. In 25 patients studied, spirometric measurements decreased. Twenty minutes after the injection, the forced vital capacity was 27% less, forced expiratory volume at 1 s 34% less and peak expiratory flow rate 15% less (all P<0.05). Right diaphragmatic excursion decreased from 4.5 cm (SD 1.2 cm) to 1.8 cm (0.6 cm) at 15 mins and to 1.1 cm (0.6 cm) at 4 h (P<0.05). Identification of the plexus by electric stimulation combined with finger compression above the injection site did not prevent diaphragmatic paresis.