River lampreys (Lampetra fluviatilis) were caught in the fall 2014 on entering the River Perhonjoki for spawning and kept at a hatchery until spawning in late spring 2015 to produce larvae for compensatory stockings. Since the lampreys died massively from early February onwards, they were investigated in March and May to clarify the cause of the deaths. The symptoms in lampreys resembled those of lipid-related thiamine (vitamin B1) deficiency of salmonines, called the M74 syndrome in the Baltic Sea area. Because the lipid content of lampreys was known to be high, thiamine concentrations were analyzed in the liver and ovulated unfertilized eggs, and the mass, length, and whole-body lipid content were also measured. The hepatic total thiamine (TotTh) concentration was significantly negatively correlated with the body lipid content and fatness index (mass to length ratio) in both females and males. In females, the hepatic TotTh concentration was less than half that in males, and the most moribund lampreys were the largest and fattiest females. Females that survived until artificial stripping of the eggs were smaller, and their hepatic thiamine concentration was higher, and the fatness index lower, than in females in March. The concentration of free thiamine in the eggs had a stronger positive correlation with the hepatic TotTh concentration than the phosphorylated thiamine derivatives and was also significantly and negatively correlated with the fatness index and mass. It was concluded that increased lipid peroxidation due to the mobilization of polyunsaturated fatty acids from lipids in exogenous vitellogenesis consumed thiamine as an antioxidant during pre-spawning fasting—especially in the fattiest females—and thus caused their death. It is suggested that to ensure compensatory stockings, the largest lampreys could be thiamine-injected at the hatchery to improve their survival until stripping of the eggs and to improve the eggs’ thiamine status.