Lay health care workers (promotores) interviewed 313 female members of remote Indian groups in northern Baja California, Mexico regarding: (1) common childhood and adult illnesses and endorsement of 'traditional' and modern therapies; (2) illness causation beliefs and knowledge of biomedical principles; and (3) the relation of ethnic identity with concepts of effective biomedical and non-biomedical therapy. The most common illnesses/symptoms reported in adults were diabetes, hypertension, high cholesterol, cold/flu, diarrhoea, low/variable blood pressure and arthritis; and in children, cold/flu, diarrhoea, bronchitis, cough, fever, empacho and dehydration. Of 285 informants, more reported at least one childhood disorder than who reported at least one adult disorder was most helped by traditional therapy [83 (29.1%) versus 44 (15.4%); P<0.0001] and both therapies [81 (28.4%) versus 42 (14.7%); P<0.001]. They reported eight naturalistic and two personalistic illness causes and manifested variable biomedical knowledge. Indian or mixed Indian/Mexican ethnic self-identity predominated, and Indian identity was unrelated to endorsement of traditional therapy. The 'biocultural synthesis' is a useful theoretical framework for viewing the findings. The Indians' pluralistic concepts have important implications for public health care workers and biomedical practitioners.