| Abstract
| - BACKGROUND: In animal studies, prolactin has been found to be important for mammaryepithelial development and its administration has been shown consistently to increase the rate ofmammary tumor formation. Previous epidemiologic studies of prolactin and breast cancer risk inpostmenopausal women have been limited in size, and the results have been inconsistent. Weconducted a nested case-control study within the prospective Nurses' Health Study cohortto better determine the relationship between plasma prolactin levels and postmenopausal breastcancer risk. METHODS: Blood samples were collected from cohort members during the periodfrom 1989 through 1990. Prolactin levels were measured by use of a microparticle enzymeimmunoassay. Included in this analysis were 306 postmenopausal women who were diagnosedwith breast cancer after blood donation but before June 1994. One or two postmenopausalcontrol subjects were matched per case subject on the basis of age, postmenopausal hormone use,and time of day and month of blood collection; the study included a total of 448 control subjects.RESULTS: In conditional logistic regression analyses, a significant positive association wasobserved between plasma level of prolactin and postmenopausal breast cancer risk (highestversus lowest quartile, multivariate relative risk = 2.03; 95% confidence interval= 1.24-3.31; two-sided P for trend = .01). The relationship wasindependent of plasma sex steroid hormone levels and was similar after excluding case subjectsdiagnosed in the first 2 years after blood collection. CONCLUSIONS: These prospective datasuggest that higher plasma prolactin levels are associated with an increased risk of breast cancerin postmenopausal women.
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