Printed articles increasingly rely on online supplements to store critical scientific information, but such data may eventually become unavailable. We checked the current availability of online supplementary scientific information published in six top-cited scientific journals (Science, Nature, Cell, New England Journal of Medicine, Lancet, Proceedings of the National Academy of Sciences USA). Here we show that in 4.7% and 9.6% of articles with online supplementary material, some of the supplements became unavailable within 2 and 5 years of their publication, respectively.