Positive associations between leukaemia and radon concentrations have been observed in England, Scotland and Wales, and Canada. Results of a similar study for the populations of 41 French administrative areas ('départements') are reported for 1984-1986. The average indoor radon and gamma ray concentrations per 'département' range from 12 to 147 Bq.m-3 and from 28 to 142 nG.h-1, respectively. Acute lymphoid leukaemia mortality rate is similar to the national level, whereas an excess of acute myeloid leukaemia deaths is observed. According to Poisson regression models and modified tests for partial correlation, acute myeloid leukaemia mortality is significantly and positively related to indoor radon concentration whether or not adjustment is made for indoor gamma ray dose, socioeconomic status and linear gradient. This result reinforces the evidence that indoor exposure to high levels of radon is a leukaemic environmental hazard.