Introduction - The objective of this study is to evaluate, for 24 cancer sites, the validity of departmental estimates of cancer incidence based on data from the Long-Term Illness (ALD) program. These estimates use the ratio of the number of ALD admissions to the number of incident cases observed in departments covered by a registry (ALD/incidence ratio). These departmental estimates are valid provided that the ratio varies little from one French department to another, or that variations in this ratio are moderate compared to the extent of geographical variations in incidence. Materials and Methods - ALD admission data were obtained from the general health insurance system, the Mutualité sociale agricole (MSA), and the Régime social des indépendants. The geographic variability of the ALD/incidence ratio was studied, adjusted for age, in the departments covered by a registry for the period 2000–2005. The geographic variability of incidence was also studied. Results - A departmental estimate of incidence is possible for eight sites: lips-mouth-pharynx, esophagus (men), stomach, colon-rectum, lung, breast, ovary, and testis. The other sites were excluded due to major geographic variability in the ALD/incidence ratio (liver, gallbladder, pancreas, larynx, melanoma, thyroid, multiple myeloma, leukemia) or because it was too high compared to geographical variations in incidence (cervix, uterine body, prostate, kidney, central nervous system). Finally, three sites were excluded due to specific coding difficulties (bladder) or insufficient sample sizes (esophagus/women, Hodgkin’s disease). Discussion - This study presents, for the first time, a systematic evaluation of the validity of departmental estimates of cancer incidence based on ALD data. This work is a first step, with the ultimate goal being to use all relevant medical-administrative data to estimate departmental and regional cancer incidence. (R.A.)