The study was aimed at determination of the realation between sickness absenteeism and medical diagnosis in the groups of employees ailing frequently and for a long time. The analysis covered 5197 randomly selected workers of textile industry in Lodz, From among them a group of 1157 employees ailing for a long time (over 30 days of sickness absenteeism annually) and a group of 888 frequently ill workers (over 3 cases of absence during that period), were selected. In about 63% of the group analysed, the physicians found at least one chronic disease. The most prevalent were: diseases of circulatory, osteo-muscular and digestive systems. The study revealed a statistically significant dependence between absenteeism due to some determined disease and corresponding to that medical diagnosis in all nosologic groups analysed--both in the case of the long ailing and frequently ill patients. The relation between the absenteeism and medical diagnosis was more pronounced in the separated groups of long and frequently ailing workers than in the whole population covered by the study. This relation was most evident in the case of digestive system diseases (Yule's coefficient = 0.89) in frequently ill persons. The values of sickness absenteeism coefficients have been the highest for those causes of absence which comply with medical diagnosis and for all nosologic units they have been markedly higher in the groups of long ailing employees. The results of the study showed, that absenteeism is a better measure of health status for long ailing and frequently ill groups of workers than for the whole population analysed.