BACKGROUND: Approximately 13% of consultations in general practice involve patients with unexplained complaints (UCs). These consultations often end with general practitioners (GPs) ordering blood tests of questionable diagnostic informativeness. OBJECTIVE: We studied factors potentially associated with GPs' decisions to order blood tests. METHODS: Cross-sectional study. Twenty-seven GPs completed registration forms after each consultation concerning newly presented UCs. RESULTS: Of the 100 analysable patients, 59 had at least one blood test ordered. The median number of ordered tests was 10 (interpercentile range [IPR-90] 2-15). Compared to abdominal complaints, the blood test ordering (BTO) probability for fatigue was five times higher (relative risk [RR] 5.2). Duration of complaints for over 4 weeks also increased this probability (RR 1.6). Factors associated with a lower BTO probability were: likelihood of background psychosocial factors (RR 0.4) and GPs having a syndrome rather than symptom type of working hypothesis (RR 0.5). CONCLUSION: We found a high rate of BTO among GPs confronted with patients with UCs. Furthermore, a considerable number of tests were ordered. The selectivity in BTO behaviour of GPs can be improved upon.