Background: Health care changes during the past decade have resulted in a greater proportion of cutaneous melanoma (CM) cases diagnosed in nonhospital settings, increasing the potential for cases to be missed by population-based cancer registries. Objective: Our purpose was to assess changes in case-finding sources in Iowa from 1977 to 1994 and to determine the extent of underreporting for the State Health Registry of Iowa, a population-based cancer registry. Methods: This study examines changing trends in the incidence of CM and compares case-finding sources (hospitals/clinics, hospital pathology laboratories, and independent pathology laboratories). A survey of dermatologists serving Iowans provides estimates of underreporting. Results: During the period 1977 to 1994, invasive CM increased 82%, whereas in situ CM increased 900%. The proportion of CM cases diagnosed in independent pathology laboratories increased to 25% of all cases. A range of 10.4% to 17.1% underreporting was estimated based on the survey of dermatologists. Conclusion: To improve the accuracy of surveillance, population-based cancer registries need to make a greater effort accessing pathology reports from nonhospital settings. (J Am Acad Dermatol 1997;37:578-85.)