Tissue typing was performed in 20 patients suffering from intermittent acute porphyria using the lymphocytotoxicity test in a two-step method. The patients belonged to different families and were not related. Compared with the healthy population, an increased frequency of HLA-A10, HLA-A29, HLA-B13, HLA-B14 and HLA-B27 was observed, whereas HLA-A2, HLA-A3, HLA-A9, HLA-A11, HLA-A28 and HLA-BW15 had decreased frequencies. No conspicuous combination between particular HLA alleles was detected. Regarding the number of differences, a relationship between the patients due to a common ancestor must be considered.