Twenty-three patients with analgesic nephropathy and apparent cessation of drug abuse were tested for blood acetaminophen and salicylate on the occasion of routine renal control examinations. In 12 patients (mean creatinine level 2.74 +/- 1.09 mg/dl) no deterioration of renal function was noted within a 1-year observation period (Group 1). In 11 patients a significant progression of renal insufficiency was observed (mean creatinine level rose from 3.86 +/- 1.06 to 6.40 +/- 3.18 mg/dl within the same observation period; Group 2). Blood tests for acetaminophen and salicylate were positive in 2 patients of Group 1 and in 9 patients of Group 2 (chi 2 = 7.326), p less than 0.01). Our data emphasize the importance of a continuous analgesic abuse hidden from the medical staff with regard to the progression of renal insufficiency in analgesic nephropathy.