STUDY QUESTION: Does culture in a closed system result in an increased number of good quality embryos (GQE) on Day 2 compared with culture in a conventional system? SUMMARY ANSWER: Culture in a closed system up to 2 days after microinjection results in similar embryo development and morphological quality compared with culture in a conventional incubation system. WHAT IS KNOWN ALREADY: Time-lapse imaging (TLI) incubators are rapidly being introduced into IVF laboratories worldwide, despite the lack of large prospective randomized trials demonstrating improvement in embryo development or pregnancy rates. STUDY DESIGN, SIZE, DURATION: A randomized controlled trial including 364 patients (365 cycles) was conducted between May 2010 and February 2014. After oocyte collection, randomization was carried out and all of a patients' oocytes were allocated to culture in either a conventional incubator or a closed incubator system in proportion 1:2 until embryo transfer on Day 2. A total of 1979 oocytes were injected and cultured in the closed system, and 1000 in the standard incubator. The primary end-point was the number of GQE in the two groups. PARTICIPANTS/MATERIALS, SETTINGS, METHODS: In total, 364 patients undergoing their first IVF cycle using ICSI, where at least one oocyte was retrieved, were randomized in a university hospital setting. Two hundred and forty patients were randomized for culture in a closed system and 124 patients for culture in the conventional incubator system (control group). Embryo assessments and final morphological scoring before transfer and cryopreservation were carried out at the same time points for embryos cultured in the conventional incubator and in the closed system. MAIN RESULTS AND THE ROLE OF CHANCE: There was no significant difference in the mean ± SD number of GQEs between groups: 2.41 ± 2.27 for the closed system group and 2.19 ± 1.82 for the control group (P = 0.34, difference 0.23, 95% confidence interval 0.69; -0.24). No significant differences were found in the number of 4-cell embryos, implantation-, pregnancy- or ongoing pregnancy rates. A significantly higher miscarriage rate was found in the TLI group compared with the control group (33.3 and 10.2%, P = 0.01). LIMITATIONS, REASONS FOR CAUTION: Culture media, temperature and gas levels were similar in the open and closed incubator systems, but different culture dishes were used. Culturing embryos for longer time period (to the blastocyst stage) may give different results. Only ICSI patients were included, which may limit the generalizability of the results. Finally, the number of GQEs on Day 2 was used as a surrogate outcome for live birth. WIDER IMPLICATIONS OF THE FINDINGS: The results are consistent with other, smaller randomized trials showing no difference in embryo quality when comparing culture in a conventional incubator with that of a closed TLI incubator system.