Five hyperlipidemic patients (one with Type III, three with Type IV, and one with Type V hyperlipoproteinemia) were found on isoelectric focusing to have both the normal isoform of apolipoprotein CII and a second isoform whose isoelectric point was consistent with a single charge change. The structure of the apolipoprotein CII variant was determined to be the same as normal apolipoprotein CII except for replacement of the normal Lys at amino acid residue 19 by Thr (C2K19T). The mutation was absent from 160 apoCII alleles screened from normolipemic subjects. The C2K19T substitution occurs in a domain of apolipoprotein CII postulated to contain a lipid-binding amphipathic alpha-helix. The presence of C2K19T in unrelated hyperlipidemic patients of various racial backgrounds suggests that, in combination with other factors such as mutations in apolipoprotein E, it plays a role in the development of hyperlipoproteinemias.