An investigation of the effects on health and social well-being of on-call shift work to cover emergencies outside working hours has been carried out by Electricité de France-Gaz de France, the French national company that supplies gas and electricity. This transversal survey compared a group of workers exposed to on-call shift work (n = 145) with a group that was not (n = 195). Each subject completed a questionnaire about the demands of his work, state of health, psychologic equilibrium using the Langner scale, and the impact of his job on social and family life. A “weekly report form,” completed for 3 consecutive weeks randomly selected using a sampling plan covering the entire year to take into account seasonal variations, was subsequently filled in by 115 of the group exposed to on-call shift work and 167 of those who were not, to investigate events occurring outside working hours and sleep. The findings show that the main demands of being on-call were telephone calls, which were five times more numerous during the weeks on-call; this was accompanied by a shorter sleeping time (a mean of 6.8 hours versus 7.4 hours during a normal week) and more frequent tiredness on waking up (25.7% vs 13.2%). No particular disorder was found more frequently in the exposed group, but the psychologic equilibrium and family and social life of the workers in the on-call shift group were disturbed: some variables in the questionnaire and indices calculated from some items revealed adverse effects. Analysis of the frequency of on-call shifts showed that these effects were greater the more frequently the subject was on-call (linear tendency test significant or nearly significant). These findings lead us to suggest an improved organization of the on-call shifts outside working hours and an extension of the intervals between on-call shifts.