The objective of this experiment was to investigate a possible interaction between protein and energy malnutrition (PEM) and intestinal nematode infections. We report on a 3×2 factorial study in which pigs were fed either a low protein energy (LPE), low protein (LP) or a normal protein energy (NPE) diet, and 6 weeks later inoculated with Trichuris suis (4000 infective eggs). Secondarily, in order to obtain a polyparasitic status, pigs were concomitantly inoculated with Ascaris suum (600 infective eggs). The number of T. suis-infected pigs was higher in LP pigs compared with NPE pigs (100 versus 58%; P = 0·037), although the differences in median T. suis worm burdens between groups at necropsy 10 weeks post-infection (p.i.) (LPE: 795; LP: 835; NPE: 48 worms; P = 0·33) were not significant. Interestingly, only T. suis in NPE were highly aggregated (k = 0·44), in contrast to a more uniform distribution among pigs in LPE (k = 1·43) and LP (k = 1·55) i.e. the majority of pigs harboured moderate worm burdens in LPE and LP, while most pigs had few or no worms in NPE. Further, T. suis worms in the LPE and LP groups were decreased in length (mean: LPE: 23·5 mm; LP: 24·3 mm; NPE: 29·4 mm; P = 0·004). The pre-patency period of T. suis was also extended in the LPE and LP groups, as reflected by lower faecal egg output at week 6 (P = 0·048) and/or 7 p.i. (P = 0·007). More A. suum worms were recovered from LP compared with the NPE group (mean: 5·4 versus 0·6; P = 0·040); this was accompanied by a higher faecal egg output in the former (P = 0·004). The low protein diets resulted in lower pig body weight gains, serum albumin, haemoglobin and packed cell volume (PCV) levels as well as diminished peripheral eosinophil counts. Infection significantly altered these parameters in the low protein groups, i.e. the pathophysiological consequences of infection were more severe in the PEM pigs. These results demonstrate that reduced protein in the diet leads to malnourishment of both the host pigs and T. suis, and compromises the pig's ability to resist infection by T. suis and A. suum.