The aim of the study was to evaluate the impact of a decision aid (DA) and its timing in women being tested for a BRCA 1/2 mutation. Women with and without a previous history of cancer were included after blood sampling for genetic testing. The DA consisted of a brochure and video providing information on screening and prophylactic surgery. To evaluate the impact of the DA, women were randomised to the DA group (n = 184), receiving the DA 2 weeks after blood sampling, or to the control group (n = 184). To evaluate the impact of timing, mutation carriers who had received the DA before the test result (n = 47) were compared to mutation carriers who received the DA after the test result (n = 42). Data were collected on well-being, treatment choice, decision and information related outcomes. The impact of the DA was measured 4 weeks after blood sampling. The impact of timing was measured 2 weeks after a positive test result. The DA had no impact on well-being, Regarding decision related outcomes, the DA group more frequently considered prophylactic surgery (P = 0.02) corroborated with higher valuations (P = 0.04). No differences were found for the other decision related outcomes. Regarding information related outcomes, the DA group felt better informed (P = 0.00), was more satisfied with the information (P = 0.00), and showed more accurate risk perceptions. Timing of the DA had no effect on any of the outcomes. No interactions were found between the DA and history of cancer. In conclusion, women being tested for a BRCA 1/2 mutation benefit from the DA on information related outcomes. Because timing had no effect, the DA is considered useful either before or after the test result.