This study investigated the effects of chronic peritoneal dialysis on thyroid function and thyroid volume of patients with chronic renal failure (CRF). We measured the levels of serum and dialysate thyroid hormones [total thyroxine (TT4), total triiodothyronine (TT3), free thyroxine (fT4), and free triiodothyronine (fT3)], thyrotropin (TSH), thyroglobulin (Tg), and thyroid volume in 10 children on chronic peritoneal dialysis [9 continuous ambulatory peritoneal dialysis (CAPD), 1 continuous cycling peritoneal dialysis (CCPD)] at baseline and after one year. Serum levels in patients were compared with those in age- and sex-matched healthy children and were scored as normal, low, or high. At the beginning of study, serum levels were low for TT3 in 1 patient, for fT3 in 8 patients, for fT4 in 3 patients, and for Tg in 1 patient; serum TSH was high in 1 patient. At the end of study, serum levels were low for TT3 in 2 patients, for TT4 in 2 patients, for fT3 in 9 patients, for fT4 in 4 patients, for TSH in 2 patients, and for Tg in 3 patients. At the start of the study, only TSH and Tg levels could be detected in peritoneal dialysate; other parameters could not be measured. One year later, levels of TSH had decreased in 6 patients and increased in 3 patients, and Tg had increased in 8 patients, compared with baseline levels. To determine the effect of CAPD, baseline results were compared with mean levels at the end of the study. Although the mean levels of all parameters, except Tg, had decreased after one year, only the decrease in serum TSH was statistically significant. On the other hand, only the levels of Tg increased significantly in peritoneal dialysate. The mean value of thyroid volume also decreased after a year, but all values were within the normal range, and the decrease was not significant. No correlation was found between dialysis duration and any parameter after one year. In conclusion, we found a decrease in serum thyroid hormones, thyroid volume, and TSH in chronic peritoneal dialysis patients. We suggest that the low TSH levels cannot be explained by loss in peritoneal dialysate and may be due to impairment of pituitary function.