['To compare the outcome of transfer of thawed blastocysts frozen on either day 5 or day 6 after in vitro fertilisation.', 'Cohort observational study.', 'Tertiary assisted conception unit in London, UK.', 'Six hundred and forty-two consecutive nondonor programmed thawed blastocyst transfer (TBT) cycles.', 'High-grade blastocysts were frozen on day 5 (n = 314) or day 6 (n = 328) after fertilisation using a slow-freezing protocol. Endometrial preparation was performed using estradiol valerate. Progesterone supplementation was commenced when the endometrial thickness had reached 7 mm or more. Frozen blastocysts were thawed on day 6 of progesterone supplementation and assessed immediately after thawing for survival, and after 3-4 hours for blastocoele re-expansion. Main outcome measures Thawed blastocyst survival and re-expansion rates, and pregnancy and live birth rates, per TBT.', 'Thawed blastocyst survival and re-expansion rates were comparable between the day 5 and day 6 groups (87% versus 87%, P = 0.50 and 73% versus 71%, P = 0.35, respectively). The live birth rate was similar between the two groups (29% versus 28.5%, P = 0.93, respectively). After adjusting for confounding variables, the odds ratio (OR) of a live birth in cycles in which the thawed blastocysts were frozen on day 6 compared with day 5 was 1.23 [95% confidence interval (CI), 0.81-1.86, P = 0.34].'] Does delayed blastocyst development influence the outcome of frozen-thawed transfer cycles?