Methods for diagnosis and surveillance of complex multi-factorial disorders such as cancer by screening of easily accessible biomarkers are disclosed. Highly stable cell free Circulating Nucleic Acids (CNA) present as both RNA and DNA species have been discovered in the blood and plasma of humans. Correlations between tumor-associated genomic/epigenetic/transcriptional changes and alterations in CNA levels are strong predictors of the utility of this biomarker class as clinical indicators. Methods for using microRNAs (miRNAs) representing a class of naturally occurring small non-coding RNAs of 19-25 nt as markers that can associate their specific expression profiles with cancer development are disclosed. Methods for isolating plasma fractions for the study of miRNA biomarkers and for measurement of circulating miRNA levels are disclosed.