Objective. To examine parents' communication with their children about the diagnosis and initial treatment of breast cancer in the mother. Design. Qualitative interview study within cross sectional cohort. Setting. Two breast cancer treatment centres. Participants. 32 women with stage I or stage II breast cancer with a total of 56 school aged children. Main outcome measures. Semistructured interview regarding timing and extent of communication with children about the diagnosis and initial treatment of the mother's illness, reasons for talking to children or withholding information, and help available and requested from health professionals. Results. Women were most likely to been talking to their children after their diagnosis had been confirmed by biopsy, but a minority waited until after surgery or said nothing at all. Family discussion did not necessarily include mention of cancer. There was considerable consistency in the reasons given for either discussing or not discussing the diagnosis. The most common reason for not communicating was avoidance of children's questions and particularly those about death. While most mothers experienced helpful discussion with a doctor concerning their illness, few were offered help with talking to children; many would have liked help, particularly the opportunity for both parents to talk to a health professional with experience in understanding and talking to children. Conclusions. Parents diagnosed with cancer or other serious illnesses should be offered help to think about whether, what, and how to tell their children and about what children can understand, especially as they may well be struggling themselves to come to terms with their illness.