This research was conducted to evaluate the effect of improvement in work environment and of segregation in a fluorescent lamp manufacturing factory. Among the total of 80 workers, 8 workers whose mercury concentration in urine reached a hazardous level ($200-299{mu}g/l$) were moved to mercury free workplace. The follow-up examination for their mercury concentration in urine was done three times ; on May 3, 1988, September 1, 1988 and April 3, 1989. The results were as follows : 1. Mercury concentration in the air was reduced from 0.140 to 0.107 $mg/m^3$ in 4 months, and to $0.087mg/m^3$ in one year after environmental improvement in workplace. However the level still exceeded the Threshold Limit Value. 2. The geometric mean of urinary mercury concentration among 80 workers was $173.0{mu}g/l;(5.1{sim}458.6{mu}g/l$). The distribution of workers according to urinary mercury concentration showed that 9 workers (11.2%) were above the mercury poisoning level ($300{mu}g/l$), 24 workers (30.0%) were $200-299{mu}g/l$, 35 workers (43.8%) were $50-199{mu}g/l$, and 12 workers (15.0%) were below 50 ${mu}g/l$. 3. Among the 24 workers whose urinary mercury concentration was 200-299 $50-199{mu}g/l$, 8 were able to be followed up. Their mean urinary mercury concentration before segregation was $244.9{mu}g/l$, but decreased to $151.4{mu}g/l$ in four months, $128.8{mu}g/l$ in six months, and $46.8{mu}g/l$ in one year after segregation.