According to data from the national Register of Occupational Diseases (Croatian Institute for Health Protection and Safety at Work), 19 cases of occupational asthma were reported in the last 9 years (2008-2016) out of the total of 2062 registered occupational diseases. This indicates insufficient diagnosing and recognition of this disorder in our working population. The aim of this paper is to describe the standard method for peak expiratory flow (PEF) monitoring through series of cases and to suggest a practical guideline for using this diagnostic tool in occupational & sport medicine practices. PEF monitoring is a relatively simple, inexpensive, non-invasive and robust method for determining the function of the respiratory system in real working conditions and environment. Current guidelines recommend PEF monitoring as an initial diagnostic tool for cases of suspected occupational asthma. A positive test indicates the association between changes in lung function and work exposure, and makes an important part in the diagnostic procedure for cases of suspected occupational asthma. However, this test cannot determine the cause of asthma because it does not distinguish occupational asthma from work-aggravated asthma, there is no standardized method for interpretation of results, and its measurements are carried out by workers themselves, so intentional and unintentional manipulations with measurements can be made. A practical guideline for usage of this diagnostic method in occupational and sport medicine practices, with special reference to the protocol of PEF measurements, presentation and interpretation of findings, is suggested as a part of the diagnostic procedure for occupational asthma.