Estrogen (ER) and progesterone (PgR) receptor content was assayed in 290 premenopausal women with primary breast cancer, in order to investigate the influence of endogenous hormones on cytoplasmic receptor concentrations throughout the menstrual cycle, subdivided into four phases of ovarian function (early and late follicular phase, early and late luteal phase). Of the total population, 231 (79.7%) patients were ER positive and 59 (20.3%) were ER negative; 220 (75.9%) were PgR positive and 70 (24.1%) were PgR negative. The percentages of positive cases were almost constant in each phase. No significant difference in mean values of ER concentration was noted throughout the cycle. Instead, the PgR concentration significantly increased from the first to the third phase (P = 0.02) and decreased from the third to the fourth phase (P = 0.01). Our results suggest that ER- and PgR- cases are homogeneously distributed and not influenced by the phase of the cycle. Moreover, they suggest that PgR measurement in the luteal phase, rather than in other phases, prevents the occurrence of false low PgR levels and, at the same time, improves its prognostic significance and response rate to endocrine therapy.