We describe an outbreak of sudden health problems in workers at a Danish grass seed plant after exposure to a particularly dusty lot of grass seeds. The seeds are called problematic seeds. The association between development of organic dust toxic syndrome (ODTS) and the handling of grass seeds causing exposure was assessed in a four-step model: (i) identification of exposure source, (ii) characterization of the emission of bioaerosols from the problematic and reference seeds, (iii) personal and stationary exposure measurement at the plant and (iv) repeated health examinations. The grass seeds were identified as the exposure source; the emissions of some bioaerosol components were up to 10(7) times higher from the problematic seeds than from reference seeds. Cleaning of the seeds was not enough to sufficiently reduce the high emission from the problematic seeds. Emission in terms of dust was 3.4 times as high from the problematic cleaned seeds as from cleaned reference seeds. The personal exposure reached 3 × 10(5) endotoxin units m(-3), 1 × 10(6) colony-forming units (cfu) of thermophilic actinomycetes m(-3), 8 × 10(5) cfu of Aspergillus fumigatus m(-3) and 9 × 10(6) hyphal fragments m(-3). Several workers working with the problematic seeds had symptoms consistent with ODTS. The most severe symptoms were found for the workers performing the tasks causing highest exposure. Respiratory airway protection proved efficient to avoid development of ODTS. Work with reference seeds did not cause workers to develop ODTS. Exposure was during work with the problematic seeds higher than suggested occupational exposure limits but lower than in studies where researchers for some minutes have repeated a single task expected to cause ODTS. In this study, many different bioaerosol components were measured during a whole working day. We cannot know, whether it is the combination of different bioaerosol components or a single component which is responsible for the development of ODTS. In conclusion, workers developed specific health symptoms due to the high bioaerosol exposure and were diagnosed with ODTS. Exposure to high concentrations of endotoxin, actinomycetes, fungi, hyphal fragments, β-glucan, and A. fumigatus occurred when working with a dusty lot of grass seed. Suspicion should be elicited by seeds stored without being properly dried and by seeds producing more dust than usually.