The BOLD signal detects changes in relative concentrations of oxy/deoxy-haemoglobin. Thus, individual blood haemoglobin levels may influence the BOLD signal-to-noise ratio in a manner independent of neural activity. In this study, we emulate group-differences in haemoglobin by performing a median split on 524 healthy elderly individuals based on individual measurements of haemoglobin. When compared, the two haemoglobin subgroups showed no differences in cognitive measures, however, significant differences in linear relationships between cognitive performance and functional connectivity were observed in four cognitive tests. Our findings confirm that haemoglobin levels are an important confounding variable in BOLD-fMRI-based studies in the elderly.