Objective - To describe the diagnostic work-up pattern in primary health care, aiming, with as few diagnostic activities as possible, to identify a number of malignancies among patients presenting with various symptoms, where a malignancy may be a differential diagnosis. Design - Survey of computerised journals. Diagnostic codes (ICD-9 system in primary health care) relating to signs, symptoms or diagnosis were selected where colorectal, pulmonary, breast and prostate malignancies might be differential diagnoses. All diagnostic actions were analysed. Subjects - 6812 patients over 30 years of age from four health centres who were recorded for a total of 14 455 selected diagnostic codes. Results - The diagnostic actions resulted in 1426 X-ray or sonographic investigations, 340 endoscopies, 16 203 haematology, clinical chemistry or microbiology tests and 667 referrals to specialists. Forty-nine malignancies were diagnosed at the primary health care centres, while 10 malignancies were classified as "missed". The frequency of faecal-occult blood tests performed was low while that of ESR and pulmonary X-ray examinations was high. Conclusion - The task for a GP identifying one or two undiagnosed malignancies per year of the four most common types among all the non-neoplastic ailments, and with as little diagnostic activity as possible, is a professional challenge to be scrutinised continuously.