The objective of this study was to determine the prevalence of occupational asthma and sensitization to workplace allergens in greenhouse flower and/or ornamental plant growers. A random sample of 40 growers who cultivated such crops and had participated in the European Farmers' Study was selected for a cross-sectional study that included (1) greenhouse characteristics and air contaminants (dust, endotoxin, and microorganisms), (2) respiratory symptoms, (3) occupational asthma diagnosed by bronchial provocation challenge, and (4) sensitization to workplace flowers or molds. Associations among respiratory symptoms, sensitization, greenhouse characteristics, and air contaminants were analyzed by logistic regression modeling. Thirty-nine growers agreed to participate (response rate, 97.5%). Bronchial provocation challenge confirmed occupational asthma in three workers (7.7%), all of them sensitized to workplace flowers or molds. No cases of occupational asthma were found among nonsensitized growers. Poor ventilation proved to be a marginal risk factor for wheezing (air velocity: odds ratio, 0.11; 95% confidence interval, 0.01-1.04). Sensitization to flowers or molds was found in 13 of 38 growers (34.2%) but was not related to greenhouse characteristics. We conclude that one-fourth of greenhouse flower and/or ornamental plant growers sensitized to workplace flowers or molds have occupational asthma, a disease that is suffered by 8% of the growers who cultivate these crops.