Convalescent plasma is frequently administered to patients with COVID-19 and has been reported, largely on the basis of observational data, to improve clinical outcomes. Minimal data are available from adequately powered randomised, controlled trials. In this double-blind, placebo-controlled, multicentre trial conducted at 12 clinical sites in Argentina and coordinated by Hospital Italiano de Buenos Aires, the authors randomly assigned hospitalised adult patients with severe COVID-19 pneumonia in a 2:1 ratio to receive convalescent plasma or placebo. The primary outcome was the patient’s clinical status 30 days after the intervention, as measured on a six-point ordinal scale ranging from total recovery to death. A total of 228 patients were assigned to receive convalescent plasma and 105 to receive placebo. The median time from the onset of symptoms to enrollment in the trial was 8 days (interquartile range, 5 to 10), and hypoxaemia was the most frequent severity criterion for enrollment. The infused convalescent plasma had a median titer of 1:3200 of total SARS-CoV-2 antibodies (interquartile range, 1:800 to 1:3200]. No patients were lost to follow-up. At day 30 day, no significant difference was noted between the convalescent plasma group and the placebo group in the distribution of clinical outcomes according to the ordinal scale (odds ratio, 0.83 (95% confidence interval [CI], 0.52 to 1.35; P=0.46). Overall mortality was 10.96% in the convalescent plasma group and 11.43% in the placebo group, for a risk difference of −0.46 percentage points (95% CI, −7.8 to 6.8). Total SARS-CoV-2 antibody titres tended to be higher in the convalescent plasma group at day 2 after the intervention. Adverse events and serious adverse events were similar in the two groups. The authors concluded that no significant differences were observed in clinical status or overall mortality between patients treated with convalescent plasma and those who received placebo.