Celery is a frequent food allergen: not only raw, but also cooked and as a spice it can produce various reactions of immediate type, from oral contact urticaria to anaphylactic shock. Most celery-allergic patients suffer from hay fever and show a skin sensitization to mugwort. An associated allergy to several spices is quite common, and therefore the term "celery-mugwort-spice-syndrome" has been proposed. The authors have investigated 35 patients, 85% of them women, in whom a current celery allergy of varying severity was diagnosed during the first 8 months of 1984. A positive skin test to celery was seen in 16 patients who denied any symptoms from it. Thorough skin testing and RAST screening with different celery preparations, other foodstuffs and mugwort-pollens were performed on all patients. The modified prick test with native celery-root proved to be the best method for detecting celery sensitization, showing a positive result in 88.6%. The scratch test with celery-salt was positive in 70.5%, intracutaneous testing with commercial extract in 63.5% and the RAST with celery-sticks in 66% of the patients. Sensitization to mugwort was absent in 8 patients only. The mugwort-sensitive group frequently showed a positive test to ragweed-pollens, a common plant in North America, and this appears to indicate cross-reactivity amongst the family of the Asteraceae. Cross-reactivity amongst the Apiaceae is the cause of the many positive results obtained with carrot, parsely, anise, fennel and caraway, the carrot allergy being of clinical importance in 50% of cases, including one with a history of anaphylactic shock after ingestion of raw carrots.(ABSTRACT TRUNCATED AT 250 WORDS)