Microneedle arrays are promising devices for the delivery of drugs and vaccines into or the skin. However, little is known about the safety of the microneedles. In this study we obtained insight in the ability of microneedles to disrupt the skin barrier, which was evaluated by transepidermal water loss (TEWL). We also determined the safety in terms of skin irritation (skin redness and blood flow) and pain sensation. We applied microneedle arrays varying in length and shape on the ventral forearms of 18 human volunteers. An effect of needle length was observed, as TEWL and redness values after treatment with solid microneedle arrays of 400 microm were significantly increased compared to 200 microm. The blood flow showed a similar trend. Needle design also had an effect. Assembled microneedle arrays induced higher TEWL values than the solid microneedle arrays, while resulting in less skin irritation. However, for all microneedles the irritation was minimal and lasted less than 2h. In conclusion, the microneedle arrays used in this study are able to overcome the barrier function of the skin in human volunteers, are painless and cause only minimal irritation. This opens the opportunity for dermal and transdermal delivery of drugs and vaccines.