The efficiency of continuous ambulatory peritoneal dialysis depends on the permeability of the peritoneal membrane. Peritoneal fibrosis (PF) causes loss of the dialytic function. Several studies have indicated that PF is closely related to the proliferation of peritoneal fibroblasts and the deposition of extracellular matrix. Transforming growth factor-beta 1 (TGF-beta1) plays a major role in stimulating extracellular matrix deposition. Frequent peritonitis occurrence may cause persistent TGF-beta1 mRNA expression. In an attempt to search for a factor related to PF, we designed a longitudinal study to measure TGF-beta1 levels in dialysate and TGF-beta1 mRNA expression in peritoneal mononuclear cells from peritoneal dialysate before onset, once a week during peritonitis, and after peritonitis in high and low peritonitis occurrence (HPO and LPO) patients. Fifteen patients with a LPO rate and 5 patients with a HPO rate were followed up longitudinally. Meanwhile, TGF-beta1 levels and TGF-beta1 mRNA expression were augmented in peritoneal dialysate effluents before, during, and after the episodes of peritonitis. The peritoneal permeability was evaluated by the peritoneal equilibration test. The results revealed that in the LPO group, TGF-beta1 and TGF-beta1 mRNA were detectable at early stages of peritonitis, but the levels decreased rapidly and were undetectable 2 weeks after peritonitis. On the other hand, in the HPO group, TGF-beta1 and TGF-beta1 mRNA persisted for a long time. We could detect TGF-beta1 and TGF-beta1 mRNA in dialysate effluents and peritoneal mononuclear cells even 2, 3, and 4 weeks after episodes of peritonitis. When compared with that of first or second episode of peritonitis, the peritoneal function evaluated with the peritoneal equilibration test was found to obviously deteriorate during the third episode of peritonitis. These findings were confirmed by an in situ hybridization technique to evaluate the relationship between TGF-beta1 mRNA expression and PF from biopsied peritoneal specimens. These findings suggest that the high TGF-beta1 levels in the dialysate are related to an increased expression of TGF-beta1 in the peritoneum. Thus, the persistent TGF-beta1 expression in the peritoneum may serve as a useful parameter in predicting PF in continuous ambulatory peritoneal dialysis patients with frequent peritonitis occurrence.