The m.3243A>G is the most prevalent pathogenic mtDNA mutation but little is known about its inheritance. We studied 34 families containing 56 mother-child relations and 82 intersibling relations to investigate its transmission. We found a significant correlation between mother and child heteroplasmy levels (r = 0.679, p < 0.001). In mothers with a heteroplasmy level of below 25% we found 30% offspring without detectable mutation, while in mothers with a heteroplasmy level of above 25%, 100% of the offspring showed the m.3243A>G mutation. Heteroplasmy levels between siblings also correlated (r = 0.512, p < ;0.001), but had limited extra predictive value because of outliers. These new data on inheritance of the m.3243A>G mutation might be of value in counseling patients and preventing transmission of the mutation.