A prospective study of 14,225 patients has been undertaken to determine the inter-relationship between periductal mastitis and duct ectasia and to establish whether there is an association between smoking and either of these two conditions. Periductal mastitis affected women at a younger age than did duct ectasia. Of 139 patients with the clinical syndrome of periductal mastitis, 97 (70 per cent) had a past history of previous periductal mastitis, compared with only one (1 per cent) of 186 patients with the clinical syndrome of duct ectasia (P < 0.0001). There was a significant excess of smokers in patients with clinically (124 (89 per cent) of 139) and pathologically (71 (91 per cent) of 78) diagnosed periductal mastitis compared with age-matched controls (both P < 0.001), but there was no such excess in those with clinically (52 (28 per cent) of 186) or pathologically (15 (23 per cent) of 64) diagnosed duct ectasia. These data suggest that periductal mastitis and duct ectasia are separate conditions which affect different age groups, have different aetiologies, and should now be considered as separate entities.