The objective of this work is to relate the performance of radiologists as measured by the probability of detecting a nodular abnormality on a chest film to the properties of the nodule and its surrounding structures measured by a microdensitometer from the film. A parameter called conspicuity has been defined. It consists of measurements of the size, shape, contrast, and the edge gradient of the nodule divided by a measurement of the complexity of the surrounding structures that tend to camouflage the nodule. Conspicuity has been measured for simulated nodules and for a series of real nodules that were originally missed in clinical practice but later detected. Correlation between film reader performance and conspicuity was found.