Hypersensitivity pneumonitis (HP) and organic dust toxic syndrome (ODTS) were discussed and compared in this review. A review of studies on the incidence of HP has demonstrated that it occurs most frequently in the winter and early spring, in adults between 60 and 70 years of age, and in nonsmokers. The prevalence of HP among farmers has been reported to be low; however, a prevalence rate of 15% or more was reported in office workers exposed to contaminated ventilation systems. Clinical symptoms of HP have included fever, myalgias, headaches, cough, and dyspnea. Elevated temperatures, increased heart and respiratory rates, and inspiratory crackles have been seen upon clinical examination. HP patients have also demonstrated leukocytosis with a left shift, hypoxia, and pulmonary function changes consistent with abnormalities in pulmonary gas exchange. Granulomas and alveolar and interstitial infiltration have been seen upon biopsy. Treatment of acute HP has involved oral glucocorticoid therapy. ODTS has occurred most commonly in workers exposed to agricultural dusts and has been thought to be 30 to 50 times more common than HP. It has been reported more often during the summer and in those between 40 and 50 years of age. ODTS has been manifested as an acute febrile illness the symptoms of which included fever, chills, myalgias, headache, dyspnea, and eye and throat irritation. Fever, tachycardia, and tachypnea have been seen upon clinical examination. Laboratory studies have demonstrated a white blood count with a left shift, occasional mild hypoxemia, and occasional changes in pulmonary function tests. Reports on the pathogenesis of HP and ODTS were reviewed. Similarities and differences between these two diseases were outlined.