Fever is sometimes associated with asthmatic reactions following specific inhalation challenges with occupational agents. Our aims were to estimate the prevalence of fever in subjects with occupational asthma confirmed by specific inhalation challenge and to examine the characteristics and clinical correlates of subjects who develop fever on specific inhalation challenge. We performed a retrospective analysis of 317 subjects who had positive specific inhalation challenge to occupational agents and a comparison of subjects who developed fever after specific inhalation challenge with a random sample of those who did not. Fifteen subjects (5%) developed fever associated with positive specific inhalation challenge. They were compared with a random sample of 60 subjects who did not develop fever. The fever group: 1) showed a larger increase in absolute number of blood neutrophils, and a larger decrease both of blood lymphocytes and forced vital capacity after specific inhalation challenge; 2) included fewer atopics; 3) had been exposed more frequently to low rather than high molecular compounds; and 4) always experienced a late reaction and had more atypical reactions. We conclude that after positive specific inhalation challenge; fever occurs infrequently, and is associated with an increase in blood neutrophils and a decrease in blood lymphocytes and forced vital capacity. Such reactions seem more likely to occur in nonatopic subjects exposed to low molecular weight agents.