The incidence of atypia and atypical ductal hyperplasia (ADH) in breast core needle biopsies varies widely (900%). I sought to identify methods to reduce the dependence of this measure on variability in the patient population. The results of all breast core needle biopsies with a diagnosis of ADH or atypia not otherwise specified for a 50-month period were reviewed. These were separated into different groups by age, and the variability of different reporting methods was compared. Of 3,026 cases, 216 were diagnosed as ADH or atypia not otherwise specified. The overall incidence of atypia by age group varied significantly from 0.029 to 0.10. The variability was reduced when atypia was expressed in relation to ductal carcinoma in situ (range, 1.0-2.1) or fibrocystic changes (range, 0.15-0.28). However, variability by age was the least when atypia was expressed in relation to the number of cases performed for calcifications (range, 0.13-0.17). Variability in atypia rates associated with age is reduced significantly when atypia is expressed in relation to the number of biopsies done for calcifications. This method of reporting atypia may allow interlaboratory comparisons with less dependence on the characteristics of the patient population.