Obsessive-compulsive disorder (OCD) is a heterogeneous disorder of unknown etiology. Phenotypic studies of affected sib-pairs (SPs) may help to characterize familial components of the phenotype. To determine whether SPs affected with OCD are similar in age at onset of obsessive-compulsive symptoms (OCS), symptom dimensions and presence of tic disorders (TDs). Forty OCD siblings ranging from 13 to 59 years old were evaluated by expert psychiatrists or psychologists. Families with two or more siblings affected with OCD were recruited from several OCD clinics in Brazil. The Yale Brown Obsessive-Compulsive Scale Checklist was used to assess OCS and the severity of OCD. The OCD diagnoses were made according to the DSM-IV. The chi-square test was used to assess concordance of TD presence within SPs based on the TD frequency reported in the literature (30%). There were significantly more siblings with early-onset OCS than with late-onset OCS (P = 0.002). Age at onset of OCS correlated positively and significantly between the two members of each SP (P = 0.005). Fourteen patients (35%) were diagnosed with TDs. There was no concordance of the TD presence within the SPs. When both were male, there was a significant sibling correlation in the contamination obsessions/cleaning compulsions dimension (ICC = 0.74; P = 0.002). Similarly, when both siblings were female, they were comparable in the hoarding obsessions/compulsions dimension (ICC = 0.76; P = 0.01). Familial factors seem to contribute to specific OCD phenotypic components such as age at onset of OCS and specific dimensions. The obvious influence of gender is as yet unexplained.