B-cell chronic lymphocytic leukemia (B-CLL) is the most common leukemia among the adult population in western countries and accounts for 30-40% of all leukemias. With survival time ranging from months to decades, the clinical course of individual CLL patients is highly variable. This heterogeneity and in the end the need for means to identify the patients with less favorable disease has encouraged the search for biomarkers that can predict the prognosis. Telomeres are repetitive structures protecting the chromosomal endings and shorten at each cell division. Telomere length (TL) has been indicated as a prognostic factor both in hematological malignancies and solid tumors. In B-CLL, TL is associated with mutation status of the immunoglobulin heavy chain variable (IGHV) gene and with clinical course. In the present thesis the main aim was to evaluate TL as a biomarker in B-CLL using a quantitative PCR-based method for TL determination. In paper I, TL was shown to be a prognostic factor for stage A and stage B/C patients, whereas IGHV mutation status predicted outcome only in stage A patients. Moreover, IGHV mutated CLL cases were subdivided by TL into two groups with different prognosis, a subdivision not seen for unmutated cases. Interestingly, the IGHV-mutated group with short telomeres had en overall survival close to that of the unmutated cases. Thus, a combination of IGHV mutation status and telomere length gave an improved subclassification of CLL identifying previously unrecognized patient groups with different outcomes. TL correlates with cellular origin of B-cell malignancies in relation to the germinal center (GC). In paper II different B-cell lymphoma/leukemia subtypes were analyzed. Shortest telomeres were found in IGHV unmutated CLLs, differing significantly from IGHV mutated cases. Contrary to this, mantle cell lymphomas (MCL) demonstrated similar TL regardless of IGHV mutation status. TL differed significantly between GC-like and non-GC-like diffuse large B-cell lymphomas (DLBCL) and follicular lymphomas (FL) had shorter telomeres than GC-like DLBCL. Hairy cell leukemias, which display Ig gene intraclonal heterogeneity, had longer telomeres than FLs and non-GC-DLBCL, but shorter than GC-DLBCL. In conclusion, TL seemed not to simply correlate with GC origin. Paper III presents a B-CLL cohort assessed for TL, genomic aberrations, IGHV mutation status, CD38 and ZAP-70 expression. An inverse correlation existed between TL and IGHV homology, CD38 and ZAP-70 expression. The presence of genomic aberrations was similar among patients regardless of TL. In contrast, 13q deletion, a favorable biomarker, was more frequent in patients with long telomeres, while 11q and 17p deletions (markers of less favorable outcome) were more frequent in the subgroup with short telomeres. In paper IV a large group of mainly indolent CLL cases from a population based cohort was studied again showing an association between TL and prognosis, especially in “good” prognosis cases as defined by other biomarkers. Multivariate analysis indicated a strong connection between IGHV mutation status, lipoprotein lipase (LPL) expression and TL. A comparison of TL in diagnostic and follow up samples demonstrated a significant correlation, and also in the follow samples TL constituted a significant biomarker for survival.