—Despite its importance for the regulation of heart function, little is known about the isoform expression of the multifunctional Ca²⁺/calmodulin-dependent protein kinase (CaMKII) in human myocardium. In this study, we investigated the spectrum of CaMKII isoforms δ₂, δ₃, δ₄, δ₈, and δ₉ in human striated muscle tissue. Isoform δ₃ is characteristically expressed in cardiac muscle. In skeletal muscle, specific expression of a new isoform termed δ₁₁ is demonstrated. Complete sequencing of human δ₂ cDNA, representing all common features of the investigated CaMKII subclass, revealed its high homology to the corresponding rat cDNA. Comparative semiquantitative reverse transcription–polymerase chain reaction analyses from left ventricular tissues of normal hearts and from patients suffering from dilated cardiomyopathy showed a significant increase in transcript levels of isoform δ₃ relative to the expression of glyceraldehyde-3-phosphate dehydrogenase in diseased hearts (101.6±11.0% versus 64.9±9.9% in the nonfailing group; P<0.05, n=6). Transcript levels of the other investigated cardiac CaMKII isoforms remained unchanged. At the protein level, by using a subclass-specific antibody, we observed a similar increase of a δ-CaMKII–specific signal (7.2±1.0 versus 3.8±0.7 optical density units in the nonfailing group; P<0.05, n=4 through 6). The diseased state of the failing hearts was confirmed by a significant increase in transcript levels for atrial natriuretic peptide (292.9±76.4% versus 40.1±3.2% in the nonfailing group; P<0.05, n=3 through 6). Our data characterize for the first time the δ-CaMKII isoform expression pattern in human hearts and demonstrate changes in this expression pattern in heart failure.