First degree relatives of alcoholics show significantly reduced P300 amplitudes. This phenomenon is though to be a vulnerability marker of alcoholism. Relatives of alcoholics with higher family loading and early onset alcoholism are at greater risk for developing alcoholism. The high risk population may comprise subtypes differentiated by family history and/or age at onset of alcoholism, with differing risks, measurable in terms of the expression of the vulnerability marker. Four groups of alcohol-naive sons/male siblings of alcoholics (n = 64) were studied. The groups were constructed on the permutation of two defining characteristics: high family loading (two or more first degree alcoholic relatives) and early onset alcohol dependence (before 25 years) in the relatives. Comparison of P300 amplitude in an auditory paradigm showed significant inter-group differences as well as progressive increase/normalization of amplitudes from the high loading-early onset group (HR1) to the low loading-late onset group (HR4), with the latter being indistinguishable from a separate group of (control) normals. There were no significant differences of P300 latency among the groups. We conclude that P300 amplitudes vary inversely with the presumed continuum of risk in those at high risk for developing alcoholism.