BACKGROUND: Diagnosing dermatomycosis from a clinical image is not always easy. Microscopy of a potassium hydroxide preparation (KOH-test) and culturing are seldomly used in general practice. Cyanoacrylate surface skin scraping (CSSS) is a new diagnostic tool that may be useful and simple. OBJECTIVES: We aimed to investigate the diagnostic value of signs and symptoms, the KOH-test and the CSSS, in patients with erythematosquamous skin lesions, using the culture as the gold standard. Our goal is to formulate an optimal algorithm for the diagnosis of mycosis, based on one or more of these tests and including both optimal accuracy and costs. METHODS: Scales from 148 consecutive general practice patients were tested using a KOH-test, CSSS and culture. Clinical data were collected using a questionnaire. RESULTS: Twenty-six (18%) positive fungal cultures were identified. The sensitivity of the clinical diagnosis was 81% and its specificity 45%; for the KOH-test, these figures were 12 and 93% respectively; and for the CSSS, 62 and 88%, respectively. The positive predictive value of the clinical diagnosis was 24% and the negative predictive value 92%; for the KOH-test these figures were 25 and 83%, respectively, and for the CSSS, 52 and 92%, respectively. Determining CSSS in all patients proved to be the most accurate policy (accuracy = 83%). The likelihood ratio of CSSS in all patients was 5.17 for a positive test result and 0.43 for a negative test result. An approach in which CSSS is obtained in only those patients whom the physician considers by clinical examination to have dermatomycosis, with no testing in other patients, results in positive and negative likelihood ratios of 4.69 and 0.56, respectively. Such a policy would result in an overall sensitivity of 50%, a specificity of 89%, a positive predictive value of 50% and a negative predictive value of 89%. DISCUSSION: The clinical picture of dermatomycosis is not very reliable. The combination of a clinical judgement if this is negative and an additional CSSS in the case of a positive clinical judgement provides us with the best cost-benefit ratio, if both diagnostic accuracy and logistic considerations are taken into consideration.