The inter-individual variation of EEG spectra is large even for the same cognitive task. We asked whether task-induced EEG spectra remain stable over more than a year. We recorded EEG in 41 healthy adults who performed a modified Sternberg task. In 20 subjects EEG was recorded in a second session with retest intervals 12-40 months. For electrodes AFz, Cz and Pz peak frequency and peak height were determined. We compared the curve shape of power spectra by regressing spectra pairwise onto each other and calculated a t-value. The t-value and pairwise differences in peak frequency and peak height between all sessions were entered into a generalized linear model (GLM) where binary output represents the recognition probability. The results were cross-validated by out-of-sample testing. Of the 40 sessions, 35 were correctly matched. The shape of power spectra contributed most to recognition. Out of all 2400 pairwise comparisons 99.3% were correct, with sensitivity 87.5% and specificity 99.5%. The intra-individual stability is high compared to the inter-individual variation. Thus, interleaved EEG-fMRI measurements are valid. Furthermore, longitudinal effects on cognitive EEG can be judged against the intra-individual variability in subjects.