OBJECTIVE: We examined, retrospectively, the relationship between the total number of oocytes retrieved and the clinical pregnancy rate, and implantation rate, in infertility treatment patients treated with their own oocytes using either conventional in vitro fertilization (IVF), or intracytoplasmic sperm injection (ICSI), techniques. A number of additional outcome measures were also examined and analysed. DESIGN: A total of 351 cycles were examined. Patients who were treated exclusively using non-donated oocytes and non-cryopreserved embryos, were selected for this study. For each patient, the total number of oocytes retrieved, the number of embryos transferred and the results of clinical pregnancy testing, were determined. For analysis, the treatment cycles examined were divided into two groups, based on whether conventional IVF or ICSI techniques were used for the insemination of oocytes (cycles where both conventional IVF and ICSI techniques were used for insemination in a single treatment cycle were excluded from analysis). The two groups were then stratified into three sub-groups, based on the number of oocytes retrieved (Sub-group I, 1-10 oocytes, Sub-group II, 11-20 oocytes, and Sub-group III, 21-40 oocytes). MATERIALS AND METHODS: Patients were treated using conventional IVF or ICSI techniques using standard methods. Statistical analysis was performed by G-test. RESULTS: For ART cases inseminated using ICSI, the highest implantation rate was seen in Sub-group II (11-20 oocytes). The implantation rates of Sub-Group I, and Sub-group III were significantly lower than that of Sub-group II (p < 0.05). A similar relationship was seen in the clinical pregnancy rates of the three sub-groups. A different relationship was seen in ART cases inseminated by conventional IVF techniques. In these cases, the highest implantation rate was seen in Sub-group III (21-40 oocytes). The implantation rates of Sub-Group I, and Sub-group II were significantly lower than that of Sub-group III (p < 0.01). A similar relationship was seen in the clinical pregnancy rates of the three sub-groups (although the differences did not reach statistical significance). CONCLUSIONS: Our study demonstrates that, at least in our hands, for patients treated using ICSI, stimulation so as to retrieve between 11-20 oocytes produced the highest implantation rate whilst, for patients treated using conventional IVF, stimulation so as to retrieve 21-40 oocytes yielded the highest implantation rate. Possible reasons for these differences are discussed.