FOUR patients with "tyrosinosis" were brought to our attention and investigated during the past year. All of them came from the secluded Lac St Jean-Saguenay area of Quebec province (Fig 1), with a population of 500,000. The principle immigration to this region occurred 150 years ago, and relatively little internal intermingling from adjacent communities has occurred since. The ancestry of the majority of this population is reputed to trace back to 21 original French-Canadian settlers. During the past decade, 41 patients with "tyrosinosis" (including the present four patients) have been discovered in 29 families who live in the region. Five of the infants are still living and have proven "tyrosinosis;" 35 infants have died with its acute manifestations;1,2 and one has died of encephalitis. The current birth rate in this region is approximately 5%; the local incidence of tyrosinosis is therefore about one in 6,000 births, or at least