Our current knowledge regarding the effects of open access to narcotic drugs is limited. This study was undertaken to clarify issues regarding availability of these drugs. Two ethnic groups in Laos were compared: the Hmong (or Meo), a tribal group with access to opium in their homes; and the Lao, a peasant people with more limited access, usually in opium dens. Of 15 demographic and clinical variables studied, 10 showed significant differences between the two cultures. Eight of these 10 could be readily accounted for by differences in drug availability. Only two differences could be ascribed to socio-ecologic factors not related to drug availability. Similarities between the two groups appeared due to (1) pharmacologic effects of narcotic addiction and (2) low social opprobrium toward addiction in both cultures. In this study, open availability of narcotic drugs appeared to favor the following: a greater proportion of female addicts; younger age of opiate usage and addiction; use of the more intoxicating route of administration; earlier onset of problems related to addiction; and shorter duration of addiction before seeking treatment.