Plasma homocysteine levels result from the effect of genetic and environmental factors. We investigated the hypothesis of familial association between folates, methylenetetrahydrofolate reductase (MTHFR) and hyperhomocysteinemia with acute events, studying three families through pedigree analysis and log-linear graphical models. In 43 subjects, 13 had homocysteine levels of >15 micromol/l. In Family A, premature venous and arterial events occurred in father and son, respectively. In Family B, several arterial premature events occurred and very high homocysteine level was found in a healthy 18-year-old nephew. In Family C, stroke occurred at the age of 16 in a boy. In all three families, all subjects with premature cardiovascular events had high homocysteine level as well as MTHFR mutation, either homozygous or heterozygous. The present results underline that hyperhomocysteinemia has a direct conditional association with cardiovascular events. Moreover, homocysteine level is a variable that links the indirect association of folates, MTHFR mutation and cardiovascular event.