The potential deleterious effects of doctors' long and arduous shifts have received relatively scant attention. This study addressed the effect of a 32 h on-call shift on 16 pre-registration medical house officers in St. James's Hospital, Dublin. We assessed 5 psychological parameters (Tension-Anxiety, Depression-Dejection, Vigour-Activity, Fatigue-Inertia and Confusion-Bewilderment) as well as 5 simple tests of alertness and concentration both pre- and post-call. The doctors were randomly assigned to be tested either pre- or post-call. On average the doctors got 4.5 hours sleep during a 32 h shift. This long shift had an adverse effect on all the psychological parameters (p < 0.05) except Depression-Dejection. The total mood disturbance score, which has been shown to correlate well with general psychological well-being, deteriorated significantly after the 32 h shift, p < 0.005. Two of the simple tests of alertness and concentration (Trail-making test and Stroop Color-Word test) also showed a significant fall-off in performance with sleep deprivation, p < 0.05, although the remaining tests (Delayed Story Recall, Critical Flicker Fusion and Three Minute Grammatical Reasoning Test) were not significantly impaired by the 32 h shift. This study shows that prolonged periods of duty without sleep adversely affect junior doctors, both in their psychological well-being and in their ability to carry out simple tasks.