Using case-control methodology, the ISICA study aims to primarily assess the relative risk of breast cancer associated with the use of individual insulins as compared to non-insulin use in patients with diabetes. Analysis will appraise individual mitigating or risk factors such as oral antidiabetic agents, type of diabetes, gestational diabetes, life-time history of BMI, reproduction related factors, oral contraceptives, hormonal replacement therapy, 1st degree relative cancer, socioeconomic status, behavioural risk, among other. Also, a number of biological parameters will be assessed (HER2, estrogens or progesterone receptors, circulating insulin). Firstly, a registry of patients with breast cancer, diagnosed with a first positive biopsy between January 2008 and June 2009 (time of alert), are identified through medical charts by a network of 88 centres across the UK, France and Canada. Secondly, case-subjects defined as those additionally suffering from diabetes, are identified. At least 1,000 of such patients are expected to meet this criterion and 750 to be included. Eligible subjects will be female, 18 years-old or more, without previous history of breast cancer, willing and able to participate. Control-subjects are defined as patients with diabetes fulfilling the same eligibility criteria and free of cancer at the time of match to cases. Controls will be independently recruited from general medical practices. An average of four controls will likely be matched to each case according to type of diabetes, age, country and region, totalling 3,000 controls. Exposure to insulin and other drugs will be collected from patients’ general practitioners/pharmacy records. All patients will be interviewed for individual risk factors. Descriptive analysis andmultivariable modelling will be done for case-control comparisons. The study results are expected in 2012.