The aim of our study was to evaluate the frequencies of MIC-A alleles and genotypes in Brazilian patients with type 1 diabetes mellitus (T1DM) and healthy controls. MHC class I chain-related gene-A (MIC-A) has been shown to be associated with susceptibility to T1DM in different populations. We analyzed the DNA samples from 86 patients and 201 healthy controls for MIC-A by PCR amplification, and fragment sizes were determined in an ABI prism DNA sequencer. We found increased frequencies of two genotypes, MIC-A 4/5.1 (pc = 0.006; OR, 6.1) and 9/9 (pc = 0.045; OR, 5.75), in our patients (mean diagnosis age, 11.70 years; SD, 8.86; mean age, 20.44 years; SD, 12.17) compared with the controls (median age, 26.41 years; SD, 8.87).