The cyclic lysosome number variation in fibroadenoma and normal breast epithelial cells in 13 women, using light microscopy and Gomory acid phosphatase method, was studied. The menstrual cycle phase was determined by the last menstrual period, plasmatic progesterone level and endometrium biopsy. Seven lumps were extirpated in the proliferative phase and other 7 in the secretory period. The lysosomes were counted in the fibroadenoma and the normal breast epithelia. The statistical analysis of the findings showed that both fibroadenoma and normal breast epithelia present a number of lysosomes statistically higher in proliferative than in the secretory phase. It was also noted that the number of lysosomes in fibroadenomas is always greater than in the normal breast, independently of the phase of the menstrual cycle.