A diagnostic biochip comprising a solid support to which one or more specific nucleic acid ligands is attached in a spatially defined manner is provided. Each nucleic acid ligand binds specifically and avidly to a particular target molecule contained within a test mixture, such as a bodily fluid. Also provided are methods for the preparation of nucleic acid ligand biochips. Further, methods for the use of the nucleic acid ligand biochip in diagnosis of a medical condition and quantitative detection of a target molecule are provided.