The angiotensin (Ang)-converting enzyme (ACE) insertion/deletion (I/D) polymorphism determines Ang II levels, but its relationship with lupus nephritis (LN) in different populations is controversial. To describe the allelic and genotypic distribution of the I/D polymorphism in Mexican mestizos with LN and assess an association with histological classes. We included 24 patients with systemic lupus erythematosus (SLE) without nephropathy, 41 with LN, 144 healthy subjects, and 36 with primary glomerulonephritis (GMN). Three ACE I/D polymorphism genotypes-ID, DD, and II--were detected by PCR using peripheral blood genomic DNA. Frequencies for II, ID, and DD were 0.29, 0.46, and 0.25 in the SLE group; 0.17, 0.63, and 0.20 in the LN group; 0.14, 0.5, and 0.36 in the GMN group; and 0.26, 0.52, and 0.22 among healthy subjects. The I/D polymorphism distribution according to histological class was class II: 1 II, 3 ID, and 1 DD; class III: 2 II, 10 ID, and 1 DD; class IV: 2 II, 9 ID, and 2 DD; class V: 2 II, 3 ID, and 4 DD; and class VI, 1 II. The histological classes with at least three patients had ID genotype as the most frequent except for class V. No association was identified between I/D polymorphisms of ACE and SLE, LN, or GMN in a Mexican population.