Most studies on comorbidity in low back pain (LBP) have been conducted in specialized settings with the use of self-reports. This study has an original design using data from family practices: the incidence of the most frequent diseases was compared in patients with and without LBP in 2004. The database includes data from 67 family physicians in 52 family practices in Flanders, Belgium. It contains data from 160,000 different patients with 1,500,000 diagnoses during the period 1994 to 2004. The incidence of the most frequent diagnoses is presented in patients with and without LBP in 2004. The diagnoses were coded according to the ICPC-2-classification.