Manifest disease symptoms of acute intermittent porphyria may be provoked by several external factors. Latent gene carriers should be identified at an early stage and informed about preventive measures. Porphobilinogen deaminase activity in red blood cells may be used as one indicator of the carrier state. However, there is an overlap of enzyme activity between healthy controls and carriers of the trait. Thus latent gene carriers cannot always be identified. In the present study a recently reported enzyme-linked immunosorbent assay (ELISA) was used to quantify the concentration of the enzyme porphobilinogen deaminase in erythrocytes in 845 individuals belonging to families with acute intermittent porphyria. Using previous available diagnostic methods 417 of them had been diagnosed as gene carriers, 339 as non-carriers, and 89 were of "uncertain" classification. Of those with "uncertain" diagnosis, 19 had a decreased concentration of porphobilinogen deaminase and could thus be diagnosed as gene carriers. However, 70 cases of the 89 were still "uncertain", which underlines the need for further improvement of the diagnostic methods.