Dihydropyrimidine dehydrogenase (DPD) is the initial and rate-limiting enzyme in the catabolism of 5-fluorouracil (5-FU), and it is suggested that patients with a partial deficiency of this enzyme are at risk of developing severe 5-FU-associated toxicity. We evaluated the importance of DPD deficiency, gender and the presence of the IVS14+1G>A mutation in the etiology of 5-FU toxicity. In 61% of cases, decreased DPD activity could be detected in peripheral blood mononuclear cells. Furthermore, the number of females (65%) in the total group of patients appeared to be higher than the number of males (35%) (p = 0.03). Patients with partial DPD deficiency appeared to have a 3.4-fold higher risk of developing grade IV neutropenia than patients with normal DPD activity. Analysis of the DPYD gene of patients suffering from grade IV neutropenia for the presence of the IVS14+1G>A mutation showed that 50% of the patients investigated were heterozygous or homozygous for the IVS14+1G>A mutation. Adopting a threshold level for DPD activity of 70% of that observed in the normal population, 14% of the population is prone to the development of severe 5-FU-associated toxicity. Below this threshold level, 90% of individuals heterozygous for a mutation in the DPYD gene can be identified. Considering the common use of 5-FU in the treatment of cancer, the severe 5-FU-related toxicities in patients with low DPD activity and the apparently high prevalence of the IVS14+1G>A mutation, screening of patients at risk before administration of 5-FU is warranted. © 2002 Wiley-Liss, Inc.