Heroin dependence is a severe and chronically relapsing substance use disorder with limited treatment options. Stress is known to increase craving and drug-taking behavior, but it is not known whether the stress hormone cortisol mediates these stress effects or whether cortisol may rather reduce craving, for example, by interfering with addiction memory. The aim of the present study was to determine the effects of cortisol administration on craving in heroin-dependent patients and to determine whether the effects depend on the daily dose of heroin consumption. We used a double-blind, placebo-controlled, cross-over study in 29 heroin-dependent patients in a stable heroin-assisted treatment setting. A single oral dose of 20 mg of cortisol or placebo was administered 105 min before the daily heroin administration. The primary outcome measure was cortisol-induced change in craving. Secondary measures included anxiety, anger and withdrawal symptoms. For the visual analog scale for craving, we found a significant interaction (P = 0.0027) between study medication and heroin-dose group (that is, daily low, medium or high dose of heroin). Cortisol administration reduced craving in patients receiving a low dose of heroin (before heroin administration: P = 0.0019; after heroin administration: P = 0.0074), but not in patients receiving a medium or high dose of heroin. In a picture-rating task with drug-related pictures, cortisol administration did not affect the ratings for the picture-characteristic craving in all the three heroin-dose groups. Cortisol also did not significantly affect secondary outcome measures. In conclusion, a single administration of cortisol leads to reduced craving in low-dose heroin addicts. The present findings might have important clinical implications with regard to understanding stress effects and regarding treatment of addiction.