What was done
Sixteen adults who had never meditated took an eight-week mindfulness course. Their brains were scanned before and after and compared with seventeen people on a waiting list. Grey-matter concentration rose in the left hippocampus, a region tied to learning and memory, and in areas linked to self-awareness and perspective.
What it does not show
Sixteen people is a small study, and the comparison group did nothing rather than something else. Later, larger studies have found smaller or mixed structural effects. Treat it as a signal, not a settled fact.
Hölzel, Carmody, Vangel, Congleton, Yerramsetti, Gard and Lazar (2011). Mindfulness practice leads to increases in regional brain gray matter density. Psychiatry Research: Neuroimaging.