The documentation of 697 cases with suspected, as suggested by regional health service units, occupational aetiology, was analysed. Within consultation those cases underwent complex audiological evaluation at the Laboratory of Audiology, using following examinations: pure tone threshold and suprathreshold audiometry, malingering tests, speech audiometry and electronystagmographic tests. The findings revealed that only 30% of subjects confirmed noise-induced hearing loss. 65.2% of those subjects exhibited cochlear localization, whereas in the remaining 34.8% the results of audiological tests showed retrocochlear impairment. The calculated mean hearing losses were characterized by symmetrical changes both in the right and left ear, as well as by directly proportional dependence on subjects' duration and exposure and age, men having more advanced hearing loss than women, despite identical mean parameters of age and exposure of both sexes. In 70% cases hearing losses were not regarded as occupational diseases, which resulted from: functional disturbances found in form of aggravation and simulation (15.3%), the presence of hearing loss lesser than the one specified by the certification rule (15.1%), post--inflammatory and adhesional changes in the middle ear (12.3%) and lack of significant exposure to noise (8.3%). It has been settled that the major causes of the diagnostic errors that we had detected were: deficient audiometric apparatus, erroneous technics of hearing measurements (among others--without masking), lack of the determination of hearing thresholds for bone conduction, drawing conclusions from single examinations, giving up simulating tests, improper evaluation of the data on occupational exposure. The considerations summing up the results of analysis include an optimum methodology of audiological studies, required standard conditions of audiometry and principles of differential diagnostics as indispensable elements of proper certification procedures in setting up the hearing impairments etiology in industrial workers.