OBJECTIVE: The aim of this study was to evaluate the effects of dietary supplementation with Agaricus sylvaticus fungus on hematological, immunological, and glycemia levels of postsurgical patients with colorectal cancer. METHODS: A randomized, placebo-controlled, clinical trial was conducted at the General Hospital of Brazil for 6 months. Fifty-six patients with colorectal cancer (stages I, II, and III) were divided into 2 groups: A. sylvaticus (30 mg/kg/day) and placebo. Complete hemogram, serum iron, and fasting glycemia evaluations were carried out throughout the treatment course. Subjects were divided according to body mass index (BMI), sex, and stage of colorectal cancer. Data were analyzed using SPSS 14.0 programs, Student's t test, and F statistical test, with P < or = 0.05 considered significant. RESULTS: After 6 months of supplementation, the group that received A. sylvaticus showed significant increases in hemoglobin (P = 0.0001), hematocrit (P = 0.0001), erythrocytes (P = 0.01), mean cell volume (P = 0.01), mean cell hemoglobin (P = 0.0001), mean cell hemoglobin concentration (P = 0.0001), and neutrophil levels (P = 0.0001). The platelet count was significantly reduced (P = 0.03), but remained within normal levels. No significant alterations were observed in the placebo group. The study group was composed of 32 women (57.1%) and 24 men (42.9%). Mean BMI was 24.65 kg/m2. Glycemia levels in the placebo group (average age 59.14 +/- 12.95 years) were: initial, 94.36 +/- 15.34 mg/dl; 3 months, 98.12 +/- 15.54 mg/dl (P = 0.03); and 6 months, 98.52 +/- 9.03 mg/dl (P = 0.01). Glycemia levels in the supplemented group (average age 56.34 +/- 15.53 years) were: initial, 95.92 +/- 11.64 mg/dl, 3 months, 94.88 +/- 12.24 mg/dl (P = 0.65); and 6 months, 92.86 +/- 6.82 mg/dl (P = 0.01). CONCLUSIONS: The results of this study suggest that supplementation with A. sylvaticus produces benefits in hematological and immunological parameters and can reduce glycemia levels in patients with colorectal cancer.