PURPOSE OF REVIEW: The rapid increases in newly recognized primary immunodeficiency diseases (PIDs), including their clinical, genetic and laboratory-associated abnormalities, make staying abreast of the latest developments a challenge. This review provides an overview of current information directly and indirectly related to the laboratory diagnosis of PIDs. RECENT FINDINGS: The latest classification and several prevalence studies provide the framework for understanding the breadth, categories and incidence rates of over 120 recognized disease entities. The latter is followed by reviews of new information related to specific PIDs including new tests, new genetic associations and newly discovered laboratory-based abnormalities. The final section presents new PIDs and a discussion of the future potential of array-based technologies in the diagnosis of PIDs. SUMMARY: The information provided in this review will allow a new appreciation of previously underestimated PIDs' prevalence rates and the delay in their diagnosis. Understanding the molecular causes of PIDs will lead to earlier diagnoses and new targets for improved therapeutic intervention. The presentation of new diagnostic tests should encourage other laboratories to assess their potential in their own laboratories. Ultimately, this information will lead to an increase in the understanding of novel laboratory parameters associated with specific PID and should improve the time required to attain an accurate diagnosis.