In a descriptive research and in a field study, we evaluated which indicators of mental workload are dominant factors in accident production in a microtomy task. The mental workload depends on the task demands and on the degree of mobilization of the worker, which is the fraction of his/her work capacity invested in the task considering itself that the degree of pleasure and satisfaction in work could vary as a function of the nature of executed task.From the hypothesis that the factors of mental workload associated to accident risks to which operators are exposed during handling of a rotative microtome represent a source of psychic suffering. The signals of psychic suffering could be related to the accomplishment of tasks considered as dangerous and many work activities could cause physical, mental, emotional and affective wearing and costs to individuals. Examining which organizational factors are related to environmental conditions and to working hours was relevant to a preventive action, as well as the dominant factors of mental workload. The research methodology involved the preparation of a questionnaire adapting the L.E.S.T. (Laboratoire d`Economie et Sociologie du Travail) method that evaluates the mental workload by four indicators, which are time constraint; complexity/ swiftness; attention; and detail. The results showed that complexity/swiftness, detail and attention indicators could be considered as effective indicators of mental workload. The indicator psychosocials factors did not presented efficacy in this analytical procedure. A reported feature is associated to accidents happening during the handling of equipment and its maintenance.