To evaluate the impact of BRCA1/2 testing and disclosure of a positive test result on women affected and unaffected with cancer. Longitudinal cohort study including women affected and unaffected with breast or ovarian cancer testing for a BRCA1/2 mutation. Data on well-being (anxiety, depression, cancer related distress, general health), treatment choice, and decision making about cancer prevention were collected at baseline (1 week after blood sampling; affected n = 192, unaffected n = 176) and at follow-up (2 weeks after disclosure of a positive test result; affected n = 23, unaffected, n = 66). Women affected and unaffected with breast or ovarian cancer were compared using univariate statistics. Change over time was examined using repeated measures analysis of variance. With respect to well-being, affected women scored worse at baseline. At follow-up, both affected and unaffected women experienced a decline in well-being, which tended to be stronger in affected women. Women diagnosed with cancer less than 1 year previously tended to report a worse well-being than those diagnosed longer ago. With respect to treatment choice, more affected women intended to obtain prophylactic surgery and valued it higher at both time points. With respect to decision making, affected women had a lower preference for participation in decision making at baseline; no differences were found at follow-up. At follow-up, both affected and unaffected women showed an increase in strength of treatment preference and a decrease in decision uncertainty. Disclosure of a positive test result had a negative impact on well-being. Affected women, especially those who have been recently diagnosed with cancer, experienced the worst well-being and could benefit from psychosocial support. (C) 2003 Wiley-Liss, Inc.