When an inherited genetic condition is diagnosed in an individual it has implications for other family members. Privacy legislation and ethical considerations can restrict health professionals from communicating directly with other family members, and so it is frequently the responsibility of the first person in a family to receive the diagnosis (the proband) to share this news. Communication of genetic information is challenging and many at-risk family members remain unaware of important information that may be relevant to their or their children's health. We conducted a randomised controlled trial in six public hospitals to assess whether a specifically designed telephone counselling intervention improved family communication about a new genetic diagnosis. Ninety-five probands/parents of probands were recruited from genetics clinics and randomised to the intervention or control group. The primary outcome measure was the difference between the proportion of at-risk relatives who contacted genetics services for information and/or genetic testing. Audit of the family genetic file after 18 months revealed that 25.6% of intervention group relatives compared with 20.9% of control group relatives made contact with genetic services (adjusted odds ratio (OR) 1.30, 95% confidence interval 0.70-2.42, P=0.40). Although no major difference was detected overall between the intervention and control groups, there was more contact in the intervention group where the genetic condition conferred a high risk to offspring (adjusted OR 24.0, 95% confidence interval 3.4-168.5, P=0.001). The increasing sophistication and scope of genetic testing makes it imperative for health professionals to consider additional ways of supporting families in communicating genetic information.