An increase in the thickness of mucous glands and in the size of the acini which compose them is a recognized feature of chronic bronchitis (Reid, 1958b), but the degree of hypertrophy which occurs has not hitherto been measured. If such increase could be readily measured, found to be constant in its relationto the presence of the disease, distinguishable from the normal and perhaps found to varywith clinical severity, a useful criterion for the pathological diagnosis of chronic bronchitis would be available. This paper describes studies which were directed to establish-ing such a yardstick. Although goblet cells in the epithelium also are increased (Florey, Carleton, and Wells, 1932; Reid, 1954), it was decided to concentrate on glands rather than goblet cells, not only because volumetrically theyare much the more significant (normally about 40 timesas great, see Appendix A), but because, especially in necropsy material, the glandsare intact while the epithelium may be damaged, precluding any count of goblet cells which, furthermore, may be so clumped in islands that their distribution may vary considerably even over small areas. Chronic bronchitis and emphysema as seen in Great Britain are usually considered to be more frequent and serious than in other countries; to some extent this may be due to difference in nomenclature which a quantitative measurement of increase in gland thickness may help to dispel by making possible a more objective assessment of the condition both here and overseas. In particular, it might explain the difference in emphasis given to the aetiological role of chronic bronchitis in relation to emphysema, and to reconcile the use of the terms" asthma" and" chronic bronchitis," in the United States and Great Britain.