BMC Cancer 2002, 2 http://www.biomedcentral.com/1471-2407/2/26 Table 1: Number of individuals are shown in relation to age at inclusion in the cohort and age at last follow up or death. Number of men at inclusion Number of men at last follow up or death 10–14 15–19 20–24 25–29 30–34 35–39 40–44 45–49 50–54 55–59 60–64 65–69 70–74 75–79 =80 that gynecomastia is more common among men who lat- er develop testicular cancer and breast cancer [5–12]. Sometimes the treatment of malignancies may be the cause for gynecomastia, such as oestrogen treatment in prostate carcinoma and cytostatic treatment in various malignancies [13–16]. The increased risk for malignant tumours with gyneco- mastia has previously been described in some case-con- trol studies. However, no earlier prospective study has been published. In order to elucidate the possible adverse or protective effect on malignancy risk of gynecomastia we formed a prospective cohort of men of whom a his- topathological diagnosis had been obtained through an operative biopsy. Methods Four hundred forty five men who had been operated for diagnostic purpose due to unilateral or bilateral gyneco- mastia in 1970–79 and of whom the pathology specimens had been sent for diagnosis to the Department of Pathol- ogy, University Hospital, Lund, Sweden, were included in the cohort. During this time period diagnostic (open) bi- opsies were very common in men with breast enlargement to exclude malignant processes. Later the operative biopsy has been replaced with fine needle biopsies. The Patholo- gy Department has an exclusive population based accrual of specimen in the South-West of the South Swedish Health Care Region. In this cohort 198 (44.5 %) men were older than 50 years and 73 (16.4 %) were younger than 20 years of age at time of diagnostic operation for gynecomastia (Table 1). 18 57 54 36 23 23 15 21 23 25 29 35 43 31 12 0 0 0 1 2 22 49 48 48 36 27 29 39 45 99 With the help of the unique personal identification number the vital status and the cancer incidence up to age 99 years of these referents were then identified from the population based Census Registry, the Cause of Death Registry and the Swedish Cancer Registry (South Swedish Regional and the National Swedish Tumour Registry). Any individual could have had more than one tumour registered. The vital status was determined up to Jan 1st, 2000. The median follow up time was 266 months, i.e. 22.2 years. No subject was lost to follow up. During the follow up time, 208 men had died and none emigrated. The observed versus expected numbers of various malig- nant tumours were then calculated using reference data from the southern health care region. Cause specific standardised incidence ratios (SIRs) and 95 % confidence intervals (CIs) were calculated. The observed number of cancer cases was assumed to follow a Poisson-distribu- tion. Hence, both 95% confidence intervals for the SIR's and the test hypothesis SIR = 1 (i.e. the cancer incidence is the same in both populations) were based on the Poisson distribution. The term "significant" refers to a p value of 0.05. All tests were two tailed. Results In table 2 the observed and expected numbers of malig- nant tumours, the SIRs and the 95 % CIs for all malignant tumours and different tumour types are shown. Men with gynecomastia had an overall risk for malignant tumours similar to that in the reference population; i.e. 68 ob- served versus 66.07 expected cases, SIR = 1.03 (95% CI 0.80–1.30). The risk for testicular cancer in the gyneco- mastia patients was significantly increased; 3 observed versus 0.52 expected, SIR = 5.82, (95% CI 1.20–17.00). Also, there were13 observed cases of squamous cell carci- Page 2 of 6 (page number not for citation purposes)