To examine the impact of the German Occupation of Guernsey (1940-1945) on breast cancer risk factors and incidence. Under study were 1019 women who stayed, or whose mothers had stayed, in Guernsey, and 1358 women evacuated or born to evacuated mothers. Amongst those born 1926-1934 who remained in Guernsey, the secular trend of earlier menarche disappeared: menarche was delayed by 12 months for those born in 1930. By March 2006, 97 breast cancers had been diagnosed, 37 in the occupied group. Unusually, higher age at menarche appeared to be associated with increased risk of breast cancer (>or=14 years vs. <or=13 years: HR = 1.52, 95% CI = 0.80-2.92). Separate analyses by birth cohort revealed a non-significantly higher incidence in the subgroup born from 1926 to 1934 (HR = 1.30, 95% CI = 0.62-2.76). Delay in menarche among women remaining in Guernsey during the Occupation, rather than being protective, was associated with an increased risk of breast cancer.