In the United States, 99% of the swine herds are infected with Mycoplasma hyopneumoniae (Mh) and the prevalence of porcine reproductive and respiratory syndrome virus (PRRSV) in U.S. swine herds is estimated at 60–80% (Zimmerman et al., 1997). These microorganisms are the two most common pathogens isolated from pigs with porcine respiratory disease complex (Thacker, 2001), and infectious disease is arguably the single most important factor affecting the performance and welfare of growing pigs. Although the growth performance and welfare of pigs with an acute infection is diminished, they can be improved by (1) reducing the severity of infection; (2) treating the symptoms of infection with, e.g., nonsteroidal anti-inflammatory drugs; and/or (3) facilitating recovery to shorten the duration of time spent clinically ill. One strategy to achieve the third aforementioned goal is to facilitate convalescence. It has been suggested that sickness behaviours including inappetence, increased sleep, and lethargy, and are part of an organized host defense strategy (Hart, 1988; Johnson, 2002). The decreased activity may better enable animals to redirect nutrient resources to support immunological defenses; it also facilitates heat conservation, which is critical for producing the beneficial febrile response. Determining how infection alters behavioural patterns is a prerequisite to assessing how current swine management practices influence convalescence. Rest is presumed to be an important part of the recovery process, but resting behaviour of pigs with an acute respiratory infection has not been assessed. Therefore, the present study reports the behaviour of pigs inoculated with Mh and PRRSV, alone or in combination. This study was part of a larger one that also determined the effects of Mh and PRRSV on growth performance, whole-body composition, and circulating inflammatory cytokines (Escobar et al., 2004). Sixty-four pigs were subjected to one of four treatment combinations (2 X 2 factorial) of Mh (inoculated at 4 weeks of age) and PRRSV (inoculated at 6 weeks of age). The four treatments were (1) control, (2) inoculation with Mh, (3) inoculation with PRRSV, and (4) inoculation with both Mh and PRRSV. Pigs that were included were video recorded during the 18 h light phase for 13 days beginning the day of PRRSV inoculation. Food intake and time spent feeding, active (standing, including walking, sitting, or feeding) and lying were determined. When pigs were lying a determination was made as to whether they were lying ventrally or laterally, and in contact with a penmate. Body temperature was measured 7 and 14 days after PRRSV inoculation. After inoculation with PRRSV, there was no significant main effect of Mh or interaction between Mh and PRRSV for food intake, body temperature, or any behaviour measured. Thus, the four treatments were pooled to form two treatments designated PRRSV negative (control and Mh; PRRSV-) and PRRSV positive (PRRSV and Mh with PRRSV; PRRSV+) and analyzed. Each day after PRRSV inoculation, PRRSV+ pigs spent less time (P = 0.005) feeding compared to PRRSV- pigs, and the decrease in feeding time was associated with a decrease in food intake (P < 0.001). PRRSV+ pigs decreased (P < 0.001) activity after inoculation with PRRSV compared to PRRSV- pigs and the amount of time spent lying was greater (P < 0.001) in PRRSV+ pigs compared to PRRSV- pigs. Furthermore, PRRSV+ pigs spent more of their total lying time in a ventral position (P = 0.06) and in contact with a penmate (P < 0.001) compared to PRRSV- pigs. Body temperature was increased (P < 0.001) in PRRSV+ pigs 7 days after PRRSV inoculation. Since sickness behaviour and fever are adaptive responses to infection, these data indicate that pigs with an acute PRRSV infection evoke a behavioural strategy that may support recovery.