Psyllium is a high molecular weight laxative that can cause occupational asthma among health care workers who prepare this product for chronically ill patients. The prevalence of immunologic sensitization and occupational asthma to psyllium is unknown. We investigated the personnel of four chronic care hospitals, including two where four cases of occupational asthma had been confirmed the year preceding the trial. A questionnaire and skin prick tests with psyllium and various inhalants were administered to 193 of 248 (78%) of the workers who agreed to participate. Seventy-five subjects (39%) had a history of respiratory symptoms in normal life and/or respiratory symptoms and/or rhinoconjunctivitis after handling psyllium. Six (3%) showed skin reactivity to psyllium. Increased specific IgE antibodies were found in 20 of 162 of the sera that were tested (12%). In the second part of the study, a histamine inhalation challenge was performed on 70 of 75 (93%) of the subjects with a history suggestive of asthma and/or occupational asthma and/or skin reactivity to psyllium. Twenty (29%) had significant bronchial hyperresponsiveness. The six subjects with a history suggestive of occupational asthma (n = 3) and/or asthma (n = 6) and/or positive skin test results to psyllium (n = 6) had significant bronchial hyperresponsiveness and increased specific IgE levels. They all underwent specific inhalation challenges with psyllium in the laboratory. Four developed bronchospastic reactions (two immediate and two dual reactions). We conclude that by including the initial four cases reported in the year preceding the initiation of this trial, the prevalence of IgE sensitization to psyllium was between 5 (skin testing) and 12% (increased specific IgE levels).(ABSTRACT TRUNCATED AT 250 WORDS)