We report herein a rare case of middle lobe syndrome, occurring in two sisters. Since selective aspirations of sputum under bronchoscopic guidance proved ineffective, a middle lobectomy was performed in both cases. Histological findings of the resected specimens showed overgrowth of the bronchial glands, lymphocytic infiltration around the alveoli and glandular metaplasia of the alveoli in both cases. Based on the allergic diathesis, we suggest that in these 2 sisters, middle lobe syndrome was caused by chronic inflammation.