The association between obstetric X-raying and childhood cancer was first identified by the Oxford Survey of Childhood Cancers in 1956. The present re-analysis exploits the case-control matching of the study while incorporating the effects of important risk determinants, notably year of birth, trimester of exposure and number of films exposed. The decline in risk over time is closely mirrored by the estimated decline in dose per film and, by constraining these two relationships to be parallel, time-invariant estimates of the extra risk per mGy are obtained. For example, it is now estimated that irradiating 10(6) foetuses with 1 mGy of X-rays would, in the absence of other causes of death, yield 175 extra cases of cancer and leukaemia in the first 15 years of life.