To examine the development of expertise in dermatology, accuracy of diagnosis and response times of subjects at five levels of expertise were assessed. A total of 100 slides, 2 typical and 3 atypical slides from each of 20 common skin disorders, were presented to six subjects at each of the following levels: second-year preclinical medical students, final year medical students, residents in family medicine, general practitioners, and dermatologists. Accuracy of diagnosis rose from 21% for medical students to 87% for dermatologists. Correct diagnosis was associated with a decrease in response time with expertise, whereas errors were associated with a dramatic increase in response time, and was slower than correct response times at all levels, suggesting that errors do not result predominantly from carelessness or speed. Typical slides accounted for a constant proportion of diagnostic errors at all higher levels of expertise, and experts continued to make a significant proportion of errors on slides shown to be relatively easy for residents. The results are shown to be at variance with any model that equates expertise with the mastery of complex rules, but they are consistent with models of expertise that propose that expertise is equated with a rapid "pattern-recognition" process, and errors result from unintended confusion with previous similar examples.