Tailored push advertising on smartphones is a key target for the advertising industry. We conducted a study with 20 professionals ‘ in the wild’: over 5 consecutive days participants received ads tailored to their personal profiles and geographical location on their personal smartphones. Of the 400 ads sent, participants accepted 20%, rejected 30%, said ‘maybe’ to 17%. Interviews revealed that accept or reject decisions were driven by specific needs at the time of delivery – e.g. a busy workload. Effective tailoring of smartphone ads requires fine-grained data on users’ emotional state and context of use – data that is sensitive and requires significant effort to obtain. Users liked context-relevant ads, but also perceived privacy costs associated with disclosing personal information. To break this conundrum, users need to be able to customize the ad service, e.g. choosing which information to disclose, when to receive ads, what types of ad.