In the advanced cancer patient, performance status has considerable prognostic power. Karnofsky performance status, together with variables reported to influence its score, was measured in advanced gastrointestinal cancer patients (n = 148). For male and female patients, age, body mass index, weight loss, triceps skinfold thickness, mid-upper arm circumference, albumin, C-reactive protein, and tumor type and stage were regressed against Karnofsky performance status. On multiple regression analysis, only mid-upper arm circumference and log10 C-reactive protein in men (r2 = 0.462, P < 0.0001) and only mid-upper arm circumference and weight loss in women (r2 = 0.485, P < 0.01) were independent predictors of Karnofsky performance status. There was a significant partial correlation, with gender as a covariable, between log10 C-reactive protein and albumin (r = -0.530, P < 0.0001) and mid-upper arm circumference (r = -0.269, P = 0.035) and weight loss (r = 0.286, P = 0.024). The results of the present study indicate that mid-upper arm circumference is a major factor that influences performance status in male and female patients with advanced gastrointestinal cancer.