Soyabean oil-based emulsions high in linoleic acid used in parenteral nutrition (PN) could interfere with immune function and may increase the risk of septic complications. Olive oil-based emulsions, high in oleic acid, could have fewer immune effects. We compared the effects of a soyabean oil-based emulsion v. an olive oil-based emulsion on infection rate, appearance of new infection episodes, leucocyte count (peak and evolution), acute-phase proteins, and major health outcomes in intensive care unit (ICU) adult patients receiving PN. The study was designed as an observational, retrospective, single-centre, cohort study in a general ICU. Patients in the SOYA cohort (n 16) received a soyabean oil-based emulsion and patients in the OLIVE cohort (n 23), an olive oil-based emulsion. Both cohorts had similar basal characteristics and received a similar energy load. The SOYA cohort received an oleic acid:linoleic acid ratio of 0·43 and the OLIVE cohort 2·99 (P < 0·001). No differences were observed in infection rate and appearance, acute-phase proteins, and major health outcomes. At the end of PN, blood leucocyte count decreased by 3·25 × 109 cells/l in the SOYA cohort and increased by 4·51 × 109 cells/l in the OLIVE cohort from baseline values (P = 0·036). Peak leucocyte count presented a trend for a higher value in the OLIVE cohort v. the SOYA cohort (18·86 v. 15·28 × 109 cells/l; P = 0·078). The use of an olive oil-based emulsion in PN had no effect on infection, acute-phase proteins, major health outcomes, and presented higher leucocyte count at the end of PN and a trend to higher peak leucocyte count when compared with soyabean oil-based emulsion in ICU patients.