The purpose of this study was to determine the effects of tumour density and tumour type on the visibility of small peripheral lung cancers on chest radiographs. We retrospectively evaluated the visibility of 63 small (< or = 20 mm) peripheral lung cancers on chest radiographs. 48 (76%) were detected in our low dose CT screening for lung cancer and 15 (24%) in routine clinical examination. Analysis was based on tumour optical contrast, gradient at the tumour margin, CT values and tumour type. There were 31 (49%) visible cancers and 32 (51%) invisible cancers on chest radiographs. Visible tumours had an optical density of 0.1-0.3 OD and a gradient of 0.03-0.11 OD mm-1. The mean size of visible tumour (14.3 mm) was larger than that of invisible tumour (11.1 mm; p < 0.001). The mean CT value (-140 HU) of visible tumour was higher than that of invisible tumour (-490 HU; p < 0.001). The detection rates of adenocarcinomas with lepidic growth (0% for type A, 29% for type B and 68% for type C) were less than those with hilic growth (100% for types D-F). All squamous and small cell carcinomas with hilic growth were visible on chest radiographs, but the numbers of each were small. In summary, tumour type influenced the contrast, gradient, CT values and margin of the tumour. Small adenocarcinomas with a lepidic tumour growth were less well seen on chest radiographs compared with small lung cancers with hilic tumour growth.