Thirty women participated in a double-blind randomized cross-over trial of the efficacy of a natural product, the powdered root of ginger (Zingiber officinale), and placebo in hyperemesis gravidarum. Three patients had to be withdrawn. Each woman swallowed capsules containing either 250 mg ginger or lactose q.i.d. during the first 4 days of the treatment period. Interrupted by a 2 days wash-out period the alternative medication was given in the second 4-day period. The severity and relief of symptoms before and after each period were evaluated by two scoring systems. The scores were used for statistical analyses of possible differences. Subjectively assessed, 19 women (70.4%) stated preference to the period in which ginger, as was later disclosed, had been given (P = 0.003). More objectively assessed by relief scores a significantly greater relief of the symptoms was found after ginger treatment compared to placebo (P = 0.035). No side effects were observed. The possible mutagenic and antimutagenic characters of ginger reported in a study of E. coli have not been evaluated with respect to any significance in humans. Powdered root of ginger in daily doses of 1 g during 4 days was better than placebo in diminishing or eliminating the symptoms of hyperemesis gravidarum.