Background: It is well established that migraine aggregates within families. Less is known about the influence of proband characteristics (e.g., age at onset, headache severity) on familial aggregation. Objective: To examine the association between the proband’ s migraine severity and age at migraine onset and familial aggregation of migraine. Methods: The authors investigated the migraine prevalence in first- degree relatives of 532 persons with migraine and control subjects in a population study. Familial aggregation was expressed as the risk of migraine in family members of probands divided by risk in control family members. Results: The relative risk (RR) of migraine in first- degree relatives of migraine probands was elevated compared with family members of controls (RR = 1.88; 95% CI: 1.30 to 2.72). The RR was also significantly higher for relatives of probands reporting onset of migraine before age 16 (2.50; 95% CI: 1.65 to 3.79) compared with those with onset at age 16 or older (1.44; 95% CI: 0.93 to 2.23). Among probands with very severe average pain scores (i.e., 9 to 10 on a 0 to 10 scale), the RR of migraine in family members was 2.38 (95% CI: 1.56 to 3.62) compared with 1.52 (0.99 to 2.34) for less severe pain (p < 0.05). Conclusion: Early onset of migraine in the proband as well as the severity of migraines are associated with higher levels of family aggregation. Less is known about the influence of proband characteristics (eg, age at onset, headache severity) on familial aggregation. Objective: To examine the association between the proband’s migraine severity and age at migraine onset and familial aggregation of migraine. Methods: The authors survey of migraine prevalence in first-degree relatives of 532 persons with migraine and control subjects in a population study. Familial aggregation was expressed as the risk of migraine in family members of probands divided by risk in control family members. Results: The relative risk (RR) of migraine in first-degree relatives of migraine probands was elevated compared with family members of controls (RR = 1.88; 95% CI: 1.30 to 2.72). The RR was also significantly higher for relatives of probands reporting onset of migraine before age 16 (2.50; 95% CI: 1.65 to 3.79) compared with those with onset at age 16 or older (95% CI: 0.93 to 2.23). Among probands with very severe average pain scores (ie, 9 to 10 on a 0 to 10 scale), the RR of migraine in family members was 2.38 3.62) compared with 1.52 (0.99 to 2.34) for less severe pain (p <0.05). Conclusion: Early onset of migraine in the proband as well as the severity of migraines are associated with higher levels of family aggregation.