To identify risk factors for incident Lyme disease in California during the period June 1, 1991 to December 31, 1992, the authors compared the activities of 101 cases of physician-diagnosed erythema migrans ascertained via both active and passive surveillance with those of 107 controls matched on sex, age, and neighborhood. Questions asked by telephone pertained to location of home, presence of wildlife around the house, hours of outdoor work and outdoor leisure activities, pet ownership, precautionary measures to avoid tick bites, tick removal methods, and knowledge about Lyme disease. For cases, activities pertained to the month prior to the onset of erythema migrans; controls were interviewed about the same activities during the same one-month period. The observation of deer and lizards around the home and a history of exposure to ticks were associated with Lyme disease (deer, odds ratio (OR) = 2.53, 95% confidence interval (CI) 1.25-5.15; lizards, OR = 2.14, 95% CI 1.14-4.04). However, the only activity associated with Lyme disease was the use for more than 5 hours per week of wide maintained trails (OR = 11.33, 95% CI 1.33-123.5); this association occurred only in persons with other outdoor leisure activities. No other behaviors or activities were identified as risk factors for acquisition of Lyme disease in California.