A genetic basis for clustering of multiple sclerosis (MS) cases, based on studies of MS families, has been proposed for decades. Few reports provide detailed neurological as well as neuroradiological findings on these patients. We report total T2-weighted intracranial lesion volumes on members of three familial MS cohorts: a mother and father with conjugal MS with one affected son and a neurologically normal son and daughter, one pair of monozygotic twin sisters with MS, and a female sibling pair with MS. We hypothesized that asymptomatic siblings in a family with two affected parents and another affected child might demonstrate clinically silent T2-weighted lesions; and that monozygotic twins with MS are more likely to express similar T2-weighted lesion volumes than non-twin sibling pairs. We found clinically silent lesions in unaffected children of the symptomatic parent couple, with a significant difference in total T2 lesion volume between these unaffected siblings and their parents, as well as their affected brother. In our other sibling pairs, T2 lesion volumes were similar between the twins and significantly different in the non-twin pair, despite similar levels of clinical functioning as determined by EDSS scoring. These results suggest that foci of demyelination might be expected in clinically normal offspring of parents with MS, possibly reflecting a genetic predisposition to subsequent development of MS.