Clinicians order next-generation genomic testing to address diagnostic uncertainty about the cause of a patient’s symptoms. Based on video-recorded observations, we examine geneticists as they return exome sequencing results to families. We find that in consultations, clinical geneticists’ interpretations of genomic findings frequently go beyond the laboratory report. The news delivery offers parents insight into the basis of clinicians’ judgment but also invites parents’ involvement in the determination of genetic causality. Through this process, clinicians and parents collaborate to define the boundaries of uncertainty. We show that through collectively managing the causal ambiguities of genomic findings, clinicians and parents enact a care relationship that works to assuage underlying feelings of diagnostic uncertainty.