The use of strong analgesics was continuously registered in 90 patients throughout 12 months via the public health authorities, who are responsible for the control of prescription of strong analgesics. After the 12 months, questionnaires were sent to the prescribing doctors about the treatment during that period. Twenty-five patients were excluded mainly due to incomplete data and non-responding GPs. Analysis of validity of the GPs' registration, with the public health authorities' registration used as the reference standard, showed 90% (95% confidence limits (CL):68-99%) agreement concerning continuing treatment with strong analgesics and 98% (CL:88-100%) regarding GPs' registration of patients not being treated with strong analgesics. In all, misclassification occurred in 5% (CL:1-13%) of the patients. Our study suggests that the GPs' information about prescriptions of strong analgesics is valid, and that it can be used in research.