The theories of decision processes and signal detection provide a framework for evaluation of observer performance. Some radiological procedures involve a search for multiple similar lesions, e.g., plain radiographic examinations for gallstones or pneumoconiosis. Presuming knowledge of the conventional receiver operating characteristic curve for detection of a single radiographic signal, a model is presented which is used to predict observer performance in an experiment requiring detection of more than 1 such signal. An experiment tests the validity of this model for detecting radiographically the presence of zero, 1, or 2 low-contrast, 2-mm diameter, Lucite beads. Results confirm the validity of the model and suggest that observer performance in relatively complex detection tasks can be predicted from simpler experiments.