other institutions (OR=2.21,95% CI: 1.10, 4.45). This result was not replicated in the present study, when applying the same surrogate measure of exposure (OR=1.1,95% CI:0.6, 2.0). The cases of the 2 studies were, however, diagnosed at different time periods: during 1960-1982 in the former study versus 1990-2007 in the present.Thus, the different results may be due to the progressive change of shift systems observed in many countries in recent decades, from traditionally slow-rotating to faster-rotating systems (3).Prior to 1970, shift schedules for Norwegian nurses often included 7-14 consecutive night shifts in addition to day shifts, and night shifts of 12 hours, duration were quite common.