In order to screen for abstruse lead poisoning in continuous ambulatory peritoneal dialysis (CAPD) patients, delta-aminolevulinic acid dehydratase (ALAD) levels were measured in 18 CAPD patients, 156 patients treated with hemodialysis (HD), and 420 control patients with normal renal function (NRF). An EDTA (ethylenediamine tetraacetic acid) mobilization test was performed in patients with low levels of ALAD (< 0.40 mumol of porphobilinogen formed per milliliter of red blood cells): 1 g of EDTA was infused IV followed by 20 L of hemofiltrate in HD patients and four bags of dialysate in CAPD patients. Lead was assayed in the ultrafiltrate liquid, the bags of dialysate, and in the 24-hour urine. ALAD levels were significantly lower in CAPD and HD patients than in the NRF subjects. ALAD was significantly correlated with EDTA mobilized lead in both dialysis and NRF patients. Using the usual criteria (EDTA mobilized lead > 800 micrograms/24 hours), the rate of lead poisoning observed was similar in the two groups. These results suggest that ALAD assay followed by the EDTA mobilization test is as effective in CAPD patients as in NRF subjects to diagnose and to treat chronic abstruse lead poisoning.