We performed a subgroup analysis of the phase III UK National Cancer Research Institute R-CHOP-14 versus R-CHOP-21 (two- versus three-weekly rituximab, cyclophosphamide, doxorubicin, vincristine and prednisolone) trial to evaluate the outcomes for 50 patients with World Health Organization 2008 classified primary mediastinal B-cell lymphoma identified from the trial database. At a median follow-up of 7·2 years the 5-year progression-free survival and overall survival was 79·8% and 83·8%, respectively. An exploratory analysis raised the possibility of a better outcome in those who received R-CHOP-14 and time intensification may still, in the rituximab era, merit testing in a randomised trial in this subgroup of patients.