Peritonitis is a complication of continuous ambulatory peritoneal dialysis (CAPD) that often causes fibrosis. Understanding the role played by the immune system is required if we are to understand the mechanisms of defense and tissue lesion triggered by germs. We compared the characteristics of the blood immunophenotypes of patients on CAPD, with and without peritonitis. This descriptive, prospective study was carried out in the dialysis unit of San José University Hospital, a tertiary care institution in Popayán, Colombia. In blood samples from 46 patients on CAPD (26 with peritonitis and 20 without peritonitis), we used flow cytometry to measure cytokine production at the single-cell level. The diagnosis of peritonitis was made by standard clinical and laboratory criteria. We noted general clinical characteristics of the patients; percentages of lymphocytes, monocytes, neutrophils, and eosinophils; and cell counts of lymphocytes (CD4 cells, CD8 cells) and their subsets [B1, B2, and natural killer (NK) cells]. We also determined the CD4:CD8 ratio. We found significant differences in the levels of serum albumin (p < 0.001), the percentages of lymphocytes (p < 0.04) and neutrophils (p < 0.04), and the counts of B1 and B2 cells, especially in patients whose CD4:CD8 ratio was below 1.2. Those patients also had more intense CD4 lymphopenia, more CD8 cells (principally T-suppressor cells), and more expansion of NK cells. In patients on CAPD, an important immune activation and rise in the percentage of B1 cells occurs that increases with peritonitis. Among the general clinical characteristics, albumin was the only one to show a statistically significant difference between patients with and without peritonitis. An important CD4 lymphopenia occurred in patients with a CD4:CD8 ratio below 1.2.