AIMS: To compare a histological hepatic iron index with a biochemical hepatic iron index, derived from atomic absorption spectroscopy measurements of hepatic iron content, for the diagnosis of genetic haemochromatosis (GH). METHODS: Histological sections of liver biopsy specimens from 70 subjects, who had previously had their biochemical hepatic iron index measured, were examined. The iron stores were scored to derive a histological hepatic iron index and were also graded from 0 to 4 by a standard grading system. The case history of each patient was then reviewed to establish a definitive clinical diagnosis and patients were classified as GH, non-GH or indeterminate. RESULTS: There were 26 cases of GH, 40 cases of non-GH and four indeterminate cases in whom a definite clinical diagnosis was not established. Using a biochemical hepatic iron index cut off level of 2.0, two cases were misclassified, with one case of GH having a biochemical hepatic iron index of 1.8 and one non-GH case having a biochemical hepatic iron index of 3.1. This could not have been improved by altering the cut off level. Using the recommended cut off level of 0.15, the histological hepatic iron index was raised in all cases of GH, but was also increased in 11 of the 40 non-GH patients. The specificity of this histological index can be improved by increasing the cut off level to 0.30. A histological iron grade of > or = 3 is more specific than the histological index but has a lower sensitivity, which particularly affects the diagnosis of younger patients with GH. CONCLUSIONS: The biochemical hepatic iron index is a reliable method for establishing a diagnosis of homozygous GH. In contrast, the histological hepatic iron index as originally described is non-specific and does not reliably distinguish patients with GH from others with a raised hepatic iron index due to other causes. The specificity of this index can be improved by increasing the cut off level used, but the discrimination provided by the histological index is still inferior to that provided by the biochemical hepatic iron index.