We developed a coding system for respiratory diseases and related conditions for computer-aided analyses of these diseases. It is a one-axis alphanumeric system with a list of three-character categories and additional digits separated by a decimal point for subcategories. Respiratory and related diseases are categorized into eight groups; diseases of (1) the lung and airway, (2) the pleura, (3) the mediastinum, (4) the diaphragm, (5) the chest wall, (6) the heart and cardiovascular system, (7) the esophagus, and (8) clinical conditions predisposing to pulmonary disease. The first character, a letter from the A to G, represents a major disease category of (1) to (7) and X that of (8); and subdivisions are shown in the second position using the numbers from 0 to 9. Further subdivisions are provided in the third position also using numbers. Actual coding was done on case reports which appeared in the Japanese Journal of Thoracic Diseases from 1973 to 1993, and all cases were coded properly. The system is expected to prove useful in computer aided analyses of respiratory diseases.