European results of assisted reproductive techniques from treatments initiated during 2000 are presented in this fourth annual report. Data were collected mainly from pre-existing national registers. From 22 countries, 569 clinics reported 279 267 cycles: IVF 126 961, ICSI 99 976, frozen embryo replacement (FER) 45 800 and oocyte donations (OD) 6530. In nine countries where all clinics reported to the register, a total of 142 174 cycles were performed in a population of 166 million, corresponding to 856 cycles per million inhabitants. After IVF and ICSI, the distribution of transfer of one, two, three and >or=4 embryos was 12.1, 46.7, 33.3 and 6.8%, respectively. Huge differences existed between countries. For IVF, the clinical pregnancy rate per aspiration and per transfer was 24.7 and 28.4%, respectively. For ICSI, the corresponding rates were 26,6% and 28,7%. These figures represent increases of 0.7 and 0.8% compared with 1999. The distribution of singleton, twin, triplet and quadruplet deliveries for IVF and ICSI combined was 73.6, 24.4, 2.0 and 0.04%. This gives a total multiple delivery rate of 26.4%. The range of triplet deliveries after IVF and ICSI ranged from 0.3 to 7.0% between countries. Compared with 1999, the number of reported cycles was increased by 8% and the clinical pregnancy rate per transfer was increased by 0.7% after IVF and by 0.8% after ICSI. The total multiple delivery rates after IVF and ICSI remain unchanged during the last 4 years.