The ILO classification of small and large opacities is the basis for the compensation of patients with pneumoconiosis. In order to validate the radiological findings, we compared the ILO classification with the gradings of pneumoconiosis in the post mortem investigation (n = 126). An ILO classification of 1/0 used as the threshold value for accepting the diagnosis of a pneumoconiosis was 100% sensitive with a specificity of 2%. With a value of 2/3 the sensitivity decreased to 60% and specificity increased to 74%. The grading of large opacities in the radiograph correlated well with the post mortem findings (rs = 0.71; p < 0.001). For accepting the diagnosis pneumoconiosis from the radiograph alone a threshold value of 1/0 is not specific. The ROC-plot revealed that specificity reached acceptable levels with an ILO classification of at least 2/3.