Forty-six cases of nosocomial infection caused by Burkholderia pickettii were reported between June and November 1993 in three metropolitan hospitals in Madrid. A case-control study of the outbreak was conducted to identify its cause. Seventy-four percent of the patients were males; the mean age +/- SD of the patients was 54 +/- 20 years. Sixty-five percent of the patients presented with some gastrointestinal disorder, and 80% had a peripheral catheter; 98% were treated with intravenous fluids, and 96% were treated with intravenous ranitidine. On the basis of results of a descriptive study and knowledge of the epidemiologic features of B. pickettii, a provisional causal hypothesis was formulated: intravenous ranitidine was the source of the outbreak. As a control measure, it was advised to stop treatment with this drug. On the basis of results of logistic regression and the microbiological isolation of B. pickettii in an ampule of the drug, we concluded that intravenous ranitidine was the cause of the outbreak.