BACKGROUND: The Mexican National Centre of Pharmacovigilance (CNFV) receives suspected adverse drug reaction (ADR) reports from the pharmaceutical industry, Federal States Centre of Pharmacovigilance (CEFV) and Healthcare Institution Centres of Pharmacovigilance (CIFV). The completeness of these suspected ADR reports is particularly important for the proper evaluation of drug safety. OBJECTIVE: The aim of the study was to evaluate the completeness of the information reported in a representative sample of suspected ADR reports submitted to the CNFV during 2007 and 2008, to evaluate the completeness of the suspected ADR reports submitted to the CNFV from different sources during these 2 years and to identify the therapeutic subgroups with the highest number of suspected ADR reports during the study years. METHODS: A cross-sectional period-prevalence study was conducted at the CNFV. Only reports of suspected ADRs submitted by the CEFV, pharmaceutical industry and CIFV during 2007 and 2008 were included in the present study (reports related to vaccines were excluded). The sample sizes to be used for each year were determined using the formula for population rate at 95% significance level. The samples for each year were randomly selected from the reports related to synthetic drugs submitted that year. The suspected ADR reports were classified according to the standing Mexican Official Norm (Norma Oficial Mexicana [NOM]) guidelines, which were used to divide the reports into four categories (0, 1, 2 and 3) based on their completeness. The seriousness of the suspected ADRs reported was also evaluated; a suspected ADR was classified as 'non-serious' when signs and symptoms are likely to be tolerated, 'moderate' when ADR is not life threatening and needs pharmacological treatment, 'serious' when ADR is life threatening and leads to hospitalization and 'fatal' when ADR contributes directly or indirectly to the patient's death. RESULTS: A total sample size of 370 and 371 suspected ADR reports from 2007 and 2008, respectively, were examined. Our analysis revealed that the pharmaceutical industry sent the highest number of suspected ADR reports for both years (58% and 63%, respectively). Results of the information completeness analysis by using the NOM categories showed that, in both study years, among the total suspected ADR reports about 32% (119) and 40% (148), respectively, were categorized as grade 0 (information insufficient to generate risk signals). Analyses of the seriousness of all suspected ADR reports revealed that 2% of reports were classified as fatal each year, whereas 6% and 5% were classified as serious and 25% and 29% were classified as moderate in 2007 and 2008, respectively. The therapeutic subgroups, according to the Anatomical Therapeutic Chemical classification, with the highest frequencies of suspected ADR reports in both study years were sex hormones and modulators of the genital system, antibacterial for systemic use, antiepileptics and psychoanaleptics, and antihypertensives. CONCLUSIONS: The completeness of the information provided in the suspected ADR reports submitted during the sample study years was incomplete and, in general, did not fulfil the requirements established by the NOM guidelines. Among the pharmaceutical industry, CEFV and CIFV, the suspected ADR reports were mainly provided by the pharmaceutical industry. It is necessary to improve the pharmacovigilance system in Mexico to achieve a high level of completeness of suspected ADR reports that totally fulfil the standing regulations.