Length at sex transition of northern shrimp (Pandalus borealis) off West Greenland decreased in the years 1991–2002. A pronounced increase in bottom temperature occurred in the middle of this period, and stock size increased substantially during the past years. Length at sex transition differed within the area and the possible effects of changing temperature and abundance were studied for five different regions on the West Greenland shelf. On average, length at sex transition declined by 1.7mm carapace length (CL) and mean bottom temperature increased by 1.9°C. The change in length at sex transition was significantly correlated with bottom temperature in three out of the five regions and for all regions combined. No clear density-dependence was detected despite of a substantial increase in northern shrimp density in parts of the area and no evidence was found that northern shrimp decrease size at sex change during periods of low female abundance to compensate for a decreased reproductive potential. The observed change in length at sex transition was apparently independent from the fishery. Earlier maturation due to higher temperature was identified as the principal cause for the decrease in the length at sex transition. Because maximum female length is regarded to be proportional to length at sex transition and female fecundity increases with size, total egg production of a given year class over its lifespan would be lower when sex transition occurs at a smaller size if not compensated otherwise, e.g. through an increase of the proportion of females that spawn in each instead of every second year.