AIMS: To evaluate how tumour size, retinal location, and patient age affect the outcome of retinoblastoma foci treated with chemotherapy. METHODS: Retrospective review of retinoblastoma foci treated with primary chemotherapy alone. Individual tumours were coded with regard to their largest basal diameter, location within the eye (macula, macula to equator, equator to ora serrata), and patient's age at diagnosis. Successfully treated tumours required no further intervention while those requiring additional treatment were coded as failures. RESULTS: 56 (72%) tumours responded successfully to chemotherapy alone while 22 (28%) required additional therapy. 26 of 31 macular tumours (84%) and 30 of 47 extramacular tumours (64%) responded to chemotherapy (p <0.060). Relative to size, 46 of 60 tumours (77%) greater than 2 mm in basal diameter were successfully treated with chemotherapy, while only 10 of 18 tumours (56%) less than or equal to 2 mm responded (p <0.088). Among the eight tumour foci diagnosed in children less than 2 months of age, five (63%) failed to respond to chemotherapy alone (p <0.032). CONCLUSION: Retinoblastoma is more likely to respond to primary chemotherapy if it is located in the macula and if the patient is older than 2 months of age. Tumours measuring less than 2 mm in diameter may be less responsive to this treatment.