Insidious neurotoxic effects of lead have been studied in a population of 42 lead-exposed workers and a reference group of 22 comparable workers with no lead exposure. The age of the individuals ranged from 18 to 50 years. The complete Wechler Adult Intelligence Scale, as well as psychomotor and memory tests, was included in the test battery. The exposure was assessed by means of the lead concentration in blood and hair and the ratio between zinc protoporphyrin and hemoglobin in the blood. Significant differences were found between the two groups of workers, especially concerning long-term memory, verbal and visuospatial abstraction, and psychomotor speed. Decreased performance in these tests was in most cases associated with indices of increased lead exposure, not only in the total population studied, but also within the lead-exposed group alone. Blood lead and zinc protoporphyrin appeared to correlate better with the intellectual impairment than did hair lead, and thus these analyses are probably better predictors for neurotoxic effects of lead. Age and exposure time were not found to be significant confounding factors in this study.