The prognosis of follicular lymphoma (FL) is significantly associated with host immunity and tumor microenvironment. Lymphopenia has been identified as a negative prognostic factor for FL. The association between monocytosis and progression-free survival (PFS) in FL remains controversial. It is unknown whether the ratio of peripheral blood absolute lymphocyte count to absolute monocyte count (ALC/AMC) at diagnosis is associated with FL prognosis. We studied 99 consecutive patients with FL who were treated with rituximab-containing chemotherapy at Kitano Hospital or Kyoto University Hospital between 2000 and 2012. We analyzed individual variables associated with the ALC/AMC ratio before treatment, as well as known prognostic factors of FL, and found that an ALC/AMC ratio of 4.7 was the best cut-off value for PFS. Kaplan-Meier analysis showed that a decreased ALC/AMC ratio was associated with inferior PFS (P = 0.022). Multivariate analysis showed that a decreased ALC/AMC ratio was a significant poor prognostic factor independent of other variables (hazard ratio, 2.714; 95 % confidence interval, 1.060-6.948; P = 0.037). The ALC/AMC ratio before treatment may be a significant prognostic factor predicting PFS of FL.