A 20-yr-old pharmaceutical worker who developed attacks of shortness of breath and wheezing 9 months after beginning work on a process in which cefadroxil powder was bottled or encapsulated will be described. Skin test with cefaxodril was negative. Baseline spirometry and methacholine inhalation test were normal. A controlled bronchial challenge test was carried out in a closed-circuit system with assessment of respirable dust concentration. Exposure to cefadroxil powder at a mean concentration of 10 mg x m(-3) for 10 min elicited an isolated immediate asthmatic response, but no response was observed to control challenge with lactose. Single-blind oral challenge test with amoxicillin up to 500 mg was well tolerated, whereas the oral challenge with cephalexin (25 mg) elicited an immediate asthmatic response. This patient had developed occupational asthma caused by inhalation of cefadroxil as confirmed by specific inhalation test. Since she tolerated oral amoxicillin, a synthetic penicillin with the side-chain identical to that of cefadroxil, it seems that she may be sensitized to the dihydrothiazine ring of cephalosporins.