Evaluating equity regarding early breast cancer detection by comparing real access to and opportunity for mammography screening according to women's social health insurance status.A retrospective follow-up study was conducted on women receiving breast cancer treatment in Bogotá, Cali, Medellin, Bucaramanga and Barranquilla between January 2005 and June 2006. A survey was carried out for collecting data about real access to and the opportunity of having mammography screening. OR and 95% confidence intervals were calculated (adjusted by multivariate logistical regression models) for establishing differences according to health insurance status. Possible interactions were investigated through verisimilarity log-like test.Women belonging to the contributory regime had a lower probability of real access to mammography screening for early detection of breast cancer than those affiliated to the subsidised regimen (OR=0,46; 0,26-0,72 95 %CI) and poor uninsured women (OR=0,36; 0,13-0,65 95 %CI). Educational level was also associated with real access to mammography, illiterate women having a lower probability of receiving mammography screening than literate women (OR=0,13; 0,02-0,30 95 %CI). Women having government-subsidised health insurance had a lower probability of accessing timely mammography screening (OR=0,10; 0,04-0,41 95 %CI).Mammography screening for the early detection of breast cancer is not equitable and such inequality particularly affects the most vulnerable women.