To estimate the effect of cost sharing on seeking care for serious and minor symptoms, we analyzed data for 3539 persons aged 17 to 61 from the Rand Health Insurance Experiment. Participants were randomly assigned to a free-care group or to insurance plans requiring them to pay part of the costs (cost-sharing group). Annual surveys were administered to determine if participants had serious and minor symptoms during the preceding month and whether they saw a physician. Serious symptoms were judged by a panel of physicians to warrant care in most instances; minor symptoms were judged neither to be severe nor to warrant care in most instances. The cost-sharing group was nearly one third less likely than the free-care group to see a physician when they had minor symptoms (6.3% compared with 9.0%; p less than 0.04). The free-care and cost-sharing groups did not differ significantly in seeking care for serious symptoms (22.3% compared with 17.9%; p = 0.095). However, for participants with low socioeconomic status who began the study in poor health, the prevalence of serious symptoms was higher in the cost-sharing than the free-care group (29.1% compared with 23.8%, p less than 0.004).