A study was carried out on a group of 95 schizophrenic patients (DSM-III-R criteria) under the age of 35, 23 of whom were cannabis abusers in the past year. The objective of the study was to evaluate the effect of cannabis on positive and negative schizophrenic symptoms, evaluated using Andreasen's Scales for the Assessment of Positive and Negative Symptoms (SAPS and SANS). There were no statistically significant differences between the groups on the SAPS; the group of cannabis abusers had higher scores except for the delusions subscale. On the SANS nonabusers scored higher, with a significant difference on the alogia subscale. The results suggest that the consumption of cannabis by schizophrenic patients could attenuate negative symptoms, which would support the self-medication hypothesis of cannabis abuse.