Many critically ill patients develop significant skeletal muscle weakness in the Intensive Care Unit (ICU), which ultimately may cause difficulties in weaning from mechanical ventilation and a protracted, expensive ICU stay. Reliable monitoring of muscle strength in this environment is difficult. The purpose of this study was to develop a reproducible, nonvolitional method of measuring adductor pollicis (AP) muscle function by magnetic stimulation of the ulnar nerve (MSUN) that could be applied to patients in the ICU and operating theater (OT). Fifty subjects (32 healthy control subjects [12 of whom were elderly], 12 ICU patients with critical illness [mean APACHE II score 20], and six otherwise healthy patients requiring minor surgery in the OT) received MSUN. In 12 of the normal subjects electrical stimulation of the ulnar nerve (ESUN) and MSUN were compared and AP twitch tension (Tw AP) and surface electromyogram (EMG) were measured. Close agreement was found between supramaximal Tw AP (median [95% CI] for MSUN 6.3 N [5-7.2 N] and ESUN 6.9 N [5.2-7.8 N] [p = NS]). Median (95% CI) values with MSUN for the 20 young and 12 elderly control subjects were 6.9 N (5. 3-7.4 N) and 7.1 N (4.4-9.8 N). Median (95% CI) Tw AP for the ICU group was 4.2 (2.2-6.7 N) and for the OT group was 5.8 (4-9.1 N). Tw AP was significantly reduced in ICU patients compared with age-matched controls (p = 0.01). MSUN can be used to measure neuromuscular function in both the laboratory and clinical settings including the ICU.