Hydroxymethylfurfural (HMF) is a heat-formed, acid-catalyzed contaminant of sugar solutions, such as inverted sugar syrup, high fructose corn syrup (HFCS) or honey, who all find their way into honey bee feeding. As HMF was noted to be toxic to adult honey bees, we investigated toxicity of HMF towards larvae. Therefore we exposed artificially reared larvae to a chronic intoxication over 6 days, with 6 different concentrations (5, 50, 750, 5000, 7500 and 10000 ppm) of HMF. Mortality was assessed from day 2 to day 7 (d7) and day 22 (d22). Concentrations ranging from 5 to 750 ppm HMF did not show an influence on larval or pupal mortality, compared to controls (p > 0.05; Kaplan-Meier analysis). Concentrations of 7500 ppm or higher caused a larval mortality of 100%. An experimental LC50 of 4280 ppm (d7) and 2424 ppm (d22) was determined. The calculated LD50 was 778 g HMF on d7 and 441 g HMF on d22. Additionally we exposed adult honey bees to high concentrations of HMF and compared them to our larval results. On d7 larvae are much more sensitive against HMF than adult honey bees after 6 days of feeding. However, measured on d22, adults show a lower LC50, which indicates that they are more sensitive than larvae. Note that d22 larvae ingest HMF with their food only for 7 days, as pupae do not consume. Hence apparently their tolerance towards HMF increases compared to adult workers. As toxicity of HMF against honey bees is a function of time and concentration, our results indicate that HMF in supplemental food will probably not cause great brood losses. Yet sublethal effects might reduce resistance against pesticides, parasites and diseases and decrease fitness of the whole colony.