Detection and discrimination of miliary disease and small pulmonary nodules were tested at six levels of spatial resolution varying between 0.3 and 2.5 l.p./mm, and receiver operating characteristic (ROC) curves were generated. The test images were obtained by digitizing selected normal and abnormal standard film/screen radiographs. Although there was no statistically significant difference in observer performance between 0.5 and 2.5 l.p./mm, the findings may have clinical relevance. Further observer detection studies of different disease processes are planned to help formulate performance standards for an electric digital radiographic device for chest imaging.