The aim of the study was to validate the Checklist Individual Strength (CIS20R) questionnaire for the assessment of prolonged fatigue in workers and to define its psychometric properties and fatigue standards for the population of Poland. The questionnaire contains 20 statements, scored on a seven-point scale (1-7), reflecting four aspects of fatigue: subjective feeling of fatigue, reduction of concentration, reduction of motivation, and reduction of physical activity. The outcome yielded by the questionnaire is expressed by the sum of all points scored in responding to all statements and mean values of its components.The study was carried out in a group (325 men and 363 women) of workers employed in industrial plants, hypermarkets and municipal services; workers of research institutes (81 men and 205 women) and pregnant women (20 persons). Based on the calculated coefficients of correlation between responses to each statement and the total score, strong discriminative power of individual statements was observed. Crombach's alpha was used to assess the consistence of the questionnaire. The coefficient value for the total score was 0.912 and for the fatigue components it ranged from 0.611 to 0.879, which indicates satisfactory reliability of the method. In addition, norms were elaborated using a sten scale. The total value of 40 indicates the borderline between low and moderate and the value of 84 between medium and high level of fatigue.The level of the risk for chronic fatigue was estimated on the basis of investigations among industrial, trade and municipal services workers. In the group of men, the necessity of prolonged standing during work, high physical effort during work, lack of fitting to physical effort during work, and stress connected with responsibility (component of occupational stress) were the risk-increasing factors. In the group of women, they were: lack of fitting to physical effort during work, noise disturbing reasoning of speech and stress connected with psychic load and lack of social support (components of occupational stress). In both groups, sleep disturbances proved to be responsible for increasing risk of the incidence of chronic fatigue. In the group of men--the diet, and in the group of women - exercise during leisure time were found to be protective factors.The study revealed that the CIS20R questionnaire is a good research tool to study chronic fatigue. It was also found that the level of fatigue in the population of Poland is much higher than that in the Netherlands, the country where the questionnaire has been developed. In the Dutch studies, the value of 76 was found to be a borderline between "normal fatigue" and fatigue occurring in people with impaired health. In our studies this value corresponds with six stens, which means that it is still within the range of moderate values.