Oyster mushroom (Pleurotus ostreatus) is one of the most widely consumed mushrooms in the world. Its spores are highly potent allergens, but their main allergen, Pleo, has so far been very rarely reported. In indoor farms, fungal spores are often found to be floating in the air. This study presents a case of a non-atopic, healthy 32-year-old woman who opened an oyster mushroom farm with her husband. During the first harvest, after 30-minute exposure, she experienced dyspnoea. Similar symptoms occurred several more times while on the farm. A month later, during packing and sorting mushrooms, after 10 min, she again felt dyspnea, accompanied by weakness, rapid pulse, and skin itching with urticaria which occurred on her forearms. Immediately after one of the exposures to oyster mushrooms, spirometry was performed. The results showed a reduced airflow obstruction. Standard skin prick tests with an inhalant and food allergens were also performed – all rendering negative results. The diagnosis also included a prick-to-prick test with oyster mushrooms (a wheal size of 12 mm), this time with a positive result. Increased total IgE (78 iu/ml) in the serum was recorded. On the basis of her medical history, reported symptoms and diagnostic tests, the patient could be diagnosed with occupational anaphylaxis and work-related asthma. A constant exposure of indoor mushroom cultivators to Pleurotus ostreatus spores increases the risk of respiratory allergy development. An environmental monitoring of oyster mushroom spores is reasonable in the industry, as it may prove useful in the prevention of the disease. Int J Occup Med Environ Health. 2021;34(4):575–9