Background and Aims Loot boxes are items in video games that may be bought for real-world money but provide ran- domized rewards. Formal similarities between loot boxes and gambling have led to concerns that they may provide a ‘gateway’ to gambling amongst children. However, the availability of loot boxes is unclear. This study aimed to determine what proportion of top-grossing video games contained loot boxes, and how many of those games were available to children. Design, setting and cases Survey of the 100 top-grossing games on both the Google Play store and the Apple App store, and the top 50 most-played games on Steam according to the data aggregator SteamSpy. Measurements The prevalence of loot boxes was measured for each platform outlined above, split by age rating. Findings A total of 58.0% of the top games on the Google Play store contained loot boxes, 59.0% of the top iPhone games contained loot boxes and 36.0% of the top games on the Steam store contained loot boxes; 93.1% of the Android games that featured loot boxes and 94.9% of the iPhone games that featured loot boxes were deemed suitable for children aged 12+. Age ratings were more conservative for desktop games. Only 38.8% of desktop games that featured loot boxes were available to children aged 12+. Conclusions Loot boxes appear to be prevalent in video games that are deemed suitable for children, especially on mobile platforms.