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Although BCL2 mutations are reported as later occurring events leading to venetoclax resistance, many other mechanisms of progression have been reported but remain poorly understood. Here we analyze longitudinal tumor samples from eleven patients with disease progression on venetoclax to characterize the clonal evolution of resistance. All patients tested showed increased in vitro resistance to venetoclax at their post-treatment timepoint. We found the previously described acquired BCL2-G101V mutation in only 4/11 patients with 2 patients showing very low variant allele fraction (VAF; 0.03-4.68%). Whole exome sequencing (WES) revealed acquired loss(8p) in 4/11 patients of which 2 patients also have gain (1q21.2-21.3) in the same cells, affecting the MCL-1 gene. In vitro experiments showed that CLL cells from the four patients with loss(8p) were more resistant to venetoclax than those without it, while the cells from two patients also carrying gain (1q21.2-21.3) showed increased sensitivity to MCL-1 inhibition. Progression samples with gain (1q21.2-21.3) were more susceptible to combination MCL-1 inhibitor with venetoclax. Differential gene expression analysis comparing bulk RNAseq data from pre-treatment and progression time points of all patients showed upregulation of proliferation, BCR and NFKB gene sets including MAPK genes. Cells from progression timepoints demonstrated upregulation of surface immunoglobulin M (sIgM) and higher pERK levels compared to the pre-timepoint, suggesting an upregulation of BCR signaling that activates the MAPK pathway. Overall, our data suggest several mechanisms of acquired resistance to venetoclax in CLL that could pave the way for rationally designed combination treatments for venetoclax resistant CLL patients.