The mechanical behaviour under uniaxial tension of polycrystalline 99.999 at.% Al sheet with a thickness of a single or a few crystallites isinvestigated experimentally. The specimens are cold rolled to a thickness (t) of 100 up to 340 micrometer and the grain size (d) is varied by recrystallisationbetween 75 and 480 micrometer. All specimens have a similar texture and a regular grain structure. For t/d <1 the flow stress varies only slightly with t/d,independently of t or d itself. For 1 <t/d <3, a strong increase in the flow stress is observed. The main explanation for this different behaviour intension, stems from the grain boundaries parallel to the specimen surface, impacting the deformation and rotation of the grains. Furthermore, it isshown that a single Hall¿Petch type relation does not apply to these kinds of specimen. Extrapolations towards specimens with columnar grainsare made through the analysis.