Ninhydrin (2,2-dihydroxy-1,3-indanedione or 1,2,3-triketo-hydrindene hydrate) is a chemical used in the detection of free amino and carboxyl groups in proteins and peptides. Allergic, immunoglobulin E (IgE)-mediated rhinitis caused by ninhydrin was diagnosed earlier in a 41 yr old woman working as a laboratory technician in a forensic laboratory. Despite handling ninhydrin only in a flow cabinet, symptoms of dyspnoea developed 6 months later. Peak flow was found to vary by 20% during working days. In the specific inhalation challenge it was shown that occupational asthma had also developed due to continuing slight exposure to ninhydrin. The titre of ninhydrin-specific IgE also increased from 0.6 to 1.1 kU x L(-1) in the follow-up. This case stresses the importance of cessation of allergen exposure in occupational allergic rhinitis, in order to prevent asthma.