Abstract: A reservoir unit is provided comprising a housing in which a reservoir with a transparent portion is arranged, the housing comprising a window allowing a portion of the reservoir to be inspected. The reservoir unit is adapted to be moved from a first “cold” condition to a second “warm” condition with no visible water condensing on the interior surface of the window portion. Further, the vent is adapted to provide pressure equalization between the interior of the housing and the exterior. In this way it is possible to perform visual inspection of the reservoir at all times without being confused by dew or condensed water drops, e.g. when a prefilled reservoir unit containing a drug which requires storage at a given low temperature, as is the case for insulin-containing drugs, is taken out in ambient temperature.