1. The association of alleles of an insertion/deletion polymorphism (I/D) of the dipeptidyl carboxypeptidase-1 gene with hypertension is controversial. If a particular allele makes a major contribution to blood pressure, then hypertensives homozygous for this allele could be expected to have higher high blood pressure than those homozygous for the alternate allele. 2. The present study examined this hypothesis by comparing pretreatment blood pressures of hypertensives who had been genotypes for the I/D polymorphism. Blood pressures for different age groups (< 50, 50-59 and > or = 60 years) were also examined for each genotype. In addition, several other parameters were examined. 3. Systolic blood pressures were found to be 167 +/- 3, 167 +/- 3 and 170 +/- 6 mmHg (mean +/- s.e.) for the genotypes II, ID and DD, respectively. Diastolic blood pressures were 113 +/- 4, 111 +/- 2 and 111 +/- 4, for the respective genotypes. One-way ANOVA showed that the respective blood pressure values did not differ significantly across genotypes. Blood pressures for different age groups of hypertensives were also similar. 4. In addition, body mass index, mean age and sex did not differ between genotypes, either for the group as a whole or for the different age groups. 5. In conclusion, the present study could find no evidence to support a genetic association between the I/D polymorphism of DCP1 and blood pressure in a group with severe, familial hypertension living in Sydney.