In large families with affective illness, identification of a biological variable is needed that reflects brain dysfunction at an earlier point than symptom development. Eye movement disorder, a possible vulnerability marker in schizophrenia, is less clearly associated with affective illness, although a subgroup of affective disorders shows smooth-pursuit eye movement disorder. The auditory P300 event-related potential may be a useful marker for risk to schizophrenia, but a role in bipolar illness is less certain. The distribution of these two biological variables and their association with symptoms in two multiply affected bipolar families is described. In a single, five-generation family identified for linkage studies through two bipolar I (BPI) probands, 128 members (including 20 spouses) were interviewed. The 108 related individuals had diagnoses of BPI (7), bipolar II (2), cyclothymia (3), or major depressive disorder (19). Eight others had generalised anxiety (1), minor depression (5), intermittent depression (1), or alcoholism (1). Sixty-nine subjects had no psychiatric diagnosis. P300 latency (81) and eye tracking (71) were recorded from a subgroup of relatives within the pedigree. Eye tracking was abnormal in 11 of 71 relatives (15.5%) and was bimodally distributed. In these 11 relatives, clinical diagnoses included minor depression (1), alcoholism (1) and generalised anxiety disorder (1). P300 latency was normally distributed and did not differ from controls. In a second family in which five of seven siblings have BPI illness, P300 latency and eye movement disorder were found in affected relatives and in some unaffected offspring. In these large families, clinical diagnoses of general anxiety, alcoholism and minor depression, when associated with eye tracking abnormality, may be considered alternative clinical manifestations of the same trait that in other relatives is expressed as bipolar illness.