BACKGROUND: The purpose of the study was to evaluate systematic reviews of research into two public health priorities, tobacco consumption and HIV infection, in terms of the reporting of data related to the applicability of trial results (i.e., whether the results of a trial can be reasonably applied or generalized to a definable group of patients in a particular setting in routine practice, also called external validity or generalisability). METHODS: All systematic reviews of interventions aimed at reducing or stopping tobacco use and treating or preventing HIV infection published in the Cochrane database of systematic reviews and in journals indexed in MEDLINE between January 1997 and December 2007 were selected. We used a standardized data abstraction form to extract data related to applicability in terms of the context of the trial, (country, centres, settings), participants (recruitment, inclusion and exclusion criteria, baseline characteristics of participants such as age, sex, ethnicity, coexisting diseases or co-morbidities, and socioeconomic status), treatment (duration, intensity/dose of treatment, timing and delivery format), and the outcomes assessment from selected reviews. RESULTS: A total of 98 systematic reviews were selected (57 Cochrane reviews and 41 non-Cochrane reviews); 49 evaluated interventions aimed at reducing or stopping tobacco use and 49 treating or preventing HIV infection. The setting of the individual studies was reported in 45 (46%) of the systematic reviews, the number of centres in 21 (21%), and the country where the trial took place in 62 (63%). Inclusion and exclusion criteria of the included studies were reported in 16 (16%) and 13 (13%) of the reviews, respectively. Baseline characteristics of participants in the included studies were described in 59 (60%) of the reviews. These characteristics concerned age in about half of the reviews, sex in 46 (47%), and ethnicity in 9 (9%).Applicability of results was discussed in 13 (13%) of the systematic reviews. The reporting was better in systematic reviews by the Cochrane Collaboration than by non-Cochrane groups. CONCLUSIONS: Our study highlighted the lack of consideration of applicability of results in systematic reviews of research into 2 public health priorities: tobacco consumption and HIV infection.