The recent interest in fibrillar biological attachment systems, as found in the gecko, has led to the development of micropatterned elastomer adhesion surfaces. All reported studies have been performed at ambient humidity neglecting its possible influence on adhesion. The present paper investigates, for the first time, the effect of systematic changes in ambient humidity from 2 to 90 %. Adhesion measurements were performed on PDMS (Sylgard 184) surfaces possessing micropillars with flat-ended and hemispherical contact shape. The pillar radius was varied between 2.5 and 25 μm; the pillar aspect ratio was kept at 1. While the adhesion of a flat sample was not affected by humidity, we found that pillar size and shape influenced the sensitivity to humidity changes: Thinner pillars, with higher pull-off forces in the dry state, exhibited decreasing adhesion values, by up to 35 %, with increasing humidity. The effect was stronger for the hemispherical tip shape, where the positive effect of finer pillars was even reversed. Possible explanations for these effects, which may lower the reliability of biomimetic adhesion devices in the presence of humidity, are given.