This year, the European Working Group on Clinical Cell Analysis (EWGCCA) organises her 5th annual conference in collaboration with the Hellenic Cytometry Society in Athens, Greece. The last 10 years have seen an explosion of novel assays for clinical cell analysis. This development is being driven by inventions such as multicolor (>4) flow cytometry, major histocompatibility complex (MHC) multimers to identify and enumerate antigen-specific T lymphocytes, and intracellular staining assays that allow the analysis of signal transduction pathways and cytokine production. As a result, phenotypic and functional cell analyses go now hand in hand (1). The overall goal of EWGCCA's Euroconferences is to provide the participants, all throughout Europe, with information on the latest developments in the field of clinical cell analysis. The emphasis is on flow cytometric assays but genomics and proteomics are also covered wherever appropriate. As such, these conferences address a broad range of topics all tied together by the common need to quantify and characterize the cells of interest. Specifically, leukemia/lymphoma immunophenotyping, antigen-specific T-cell responses in cancer, viral diseases, vaccination and autoimmunity, CD20 antibody-targeted therapy, circulating endothelial cells as biomarkers for cancer and vascular disease, as well as the use of flow cytometry for allergy diagnosis and the analysis of multiplexed arrays and intracellular signaling processes are addressed at the 5th Euroconference. This conference is concluded by a workshop on controversial areas in the field of clinical cell analysis, including affordable vs. comprehensive methods for CD4+ T-cell counting, cell enrichment vs. flow cytometry for circulating endothelial cell detection, minimal vs. comprehensive panels for leukemia/lymphoma immunophenotyping, and quantitative flow cytometry with ZAP-70 expression as a case in point. For the first time, the conference is preceded by a 2-day practical training course in clinical cytometry, which is inspired by the successful series of annual courses by the Clinical Cytometry Society. The European Working Group on Clinical Cell Analysis (EWGCCA) was established in 1996 in order to establish collaboration between large European centers using (flow) cytometric cell analysis for clinical purposes. The aims of EWGCCA are the advancement and standardization of techniques and assays used for clinical cell analysis. These techniques encompass the analytical characterization of intact cells (using flow cytometry and/or quantitative microscopy) as well as the analytical characterization of genes and gene expression (using molecular techniques). EWGCCA seeks to achieve its goals through the organisation of annual Euroconferences and workshops, through the conduction of multicenter standardization trials, and through the publication of guidelines (2, 3). In 2002, EWGCCA was established as a nonprofit foundation in The Netherlands. Thus far, some 40 laboratories have participated to multicenter standardization trials organised by EWGCCA. The objectives of the Hellenic Cytometry Society are: (i) to promote in Greece the scientific collaboration in the field of flow cytometry by organising scientific meetings; (ii) to exchange information with other (inter)national organisations in the same field; (iii) to advance the quality in cytometry by recommending internal and external quality control procedures, exchanging reagents for standardization and establishing technical recommendations, as well as recommendations to achieve accreditation; (iv) to advance education in all fields and levels of cytometry; (v) to stimulate scientific collaboration between cytometry users and manufacturers of products for cytometry. As scientific organizing committee, we are very pleased with the endorsement and financial support of the International Society for Analytical Cytology (ISAC) to the 5th Euroconference, and with the opportunity offered by Clinical Cytometry to publish the 45 peer-reviewed and selected abstracts that have been submitted to us before 1 July 2005. These are shown in the following section.